Why is he called Ayyappan ?

Before answering this question we can first talk about the naadis , that are energy channels within our body.

In the Pranamayakosha or the energy body, there are 72,000 nadis. The 72,000 nadis spring from three basic nadis – the left, the right and the central – the Ida, Pingala, and Sushumna.

The Ida and Pingala represent the basic duality in the existence. It is this duality which we traditionally personify as Shiva and Shakti. 

Pingala

• Pingala is related to Shiva.
• Pingala is the extroverted, solar nadi, and corresponds to left hemisphere of the brain. 
• Pingala na
• controls all the vital processes.

Ida  

• Ida is related to Shakthi.
• Ida is the introverted, lunar nadi, and refers to the right hemisphere of the brain.
•  Ida nadi controls all the mental processes.

The Sushumna is the central energy channel or Nadi that travels the full length of the middle of the spinal cord. It is the channel through which pranic energy flows and through which we experience Kundalini Awakening.

Mahavishnu

Now about the relationship between Shakthi and Vishnu.

Also, according to Vayaviya Samhita of Shiva Mahapurana Lord Shiva alone is Purusha while all others, including Brahma, Vishnu and Rudra are Prakriti Amshas. 

As per Shakta Mahabhagavatam, Srimad Dakshina Kalika Herself incarnates in Lohita Kalpa as Vasudeva Sri Krishna. 

Todala Tantra equates Dasa Mahavidyas with Dasa Avataras and it is said that Lord Vishnu's ten Avataras are actually Amshas of Dasa Mahavidya, and Lord Krishna is born as Amsha of Adi Mahavidya Bhagavati Dakshina Kalika. Similar to this, Lalita Sahasranama Stotram and Lalitopakhyanam section of Brahmanda Purana invoke Lalitambika as 'karAngulI nakhotpannA nArAyanasya daSAkruti' (one who creates Narayana's ten Avatars from Her finger nails).

Also, Klim is Kama Bija, and it is said in Dakini Tantra that personification of Klim Bija is Kameshvari (Lalita Tripurasundari). She then becomes Krim Bija (Samhara Bija) and is seen as Supreme Kalika. In order to make the coexistence of Krim & Klim possible, Kameshvari also becomes Krishna. This is emphasised more in Tantraraja Tantra, where Lalitambika Herself transforms into Lord Krishna and sports with Gopikas.
Remember, Klim Bija denotes Kameshvari and the same Bijakshara is also used in Krishna Mantras. E.g; om klIm krShNAya govindAya gopIjana vallabhAya. Why Bijakshara of Lalitambika be used in Krishna Mantra unless both are one?

Evidence from vedas:

“rudraatpravartate biijaM biijayonirjanaardanaH |” (Rudra Hridayopanishad 8)
“Rudra is the generator of the seed. Vishnu is the embryo of the seed

Then RV 7:40:5 says “Rudra derives his Rudra strength from Vishnu”.

Rudra derives strength from Vishnu as he is his Shakthi. From which we again understand that Vishnu and Shakthi are the same. Vishnu is the male form of Aadi Shakthi.

Since that is answer is cleared we can continue.

Lord Ayyappa is said to the son of Shiva and Vishnu ( Mohini form). When we analyse it with the above knowledge of the Pingala and ida concept, we see that Ayyappan is related to sushumna, that is the union of Ida and Pingala. 

We can see a picture showing the 5 basic elements in the human body.

Square symobol represents earth ,Crescent represents water,  Inverted Triangle represents Fire, Star represents Vayu and huge oval represents Akasha.

• Akasha- field - blue (colorless light, including all of colours),
• Vayu - air – between the green and dark blue,
• Agni - fire - red,
• Apas - water - white,
• Bhumi (Prithivi) - earth - yellow.

We know that our body and whole of creation is made from the combination of these basic elements that is shown in the Sushumna channel in the above drawing. 

There are six important Chakras: Muladhara, Svadhisthana, Manipura, Anahata, Vishuddha, and Ajna. Sahasrara is the chief Chakra. It is in the head.

 These 7 Chakras correspond to the Lokas (Bhuh, Bhuvah, Svah, Maha, Jana, Tapa, and Satya Lokas).

 Muladhara and above are the centres of Pancha Bhutas (five elements): earth, water, fire, air and ether.

 "Lord Ayyappa is the lord of the Pancha bhutas. He is above these five elements which is seen from Muladhara and above... that is why he is called Ayy ( means 5) and appan ( Father or Lord )".

When one does sadhanas and open his sushumna naadi and raise his energies upwards through the Sushumna he will see the great Lord Ayyappan. They he will explain to him the real meaning of "Tat tvam asi ".

The glorious temple of Lord Ayyappan, Sabarimala has the word Tat va masi written in front of it.

What is Tat tvam asi ?

That thou art, That art thou, You are that, or That you are

That is what is in me...is in you and so we both are the same.  Absolute equality of 'tat', the Ultimate Reality, Brahman, and 'tvam', the Self, Atman. In simple sentence, we can say it is... " The creator and the creation are the same ".

  There is a soul stirring prayer in the Shaivite tradition which echoes the words "Shivoham Shivoham....." which means "I am Shiva" or "Shiva is present in me". Tat tvam asi is a similar concept.

Austerities followed during Mandala Vratham

• Waking up early and having a bath.
• Many people visit a nearby shrine or pray at home.
• Most devotees take food only after bath.
• Non-vegetarian food is totally avoided during the 41 day period.
• Devotees try to consume freshly cooked vegetarian food.
• Evening bath and visiting a shrine or puja at home is performed.
• Devotees observe Brahmacharyam (no sex or celibacy) during the vratham period.

 Each devotee taking the vrata is called Swami Ayyappa and is respected as God in our tradition as he is in the extreme state of purity. 

Those devotees going to the Sabarimala temple apart from observing the austerities wear a mala made from Rudrakhsam or Tulsi beeds. This ritual is known as Maladharan. It is performed at a temple or in one’s own house after performing a simple puja. The garland is received from a temple priest or from a guruswami or from an elderly person amidst chanting of Swami Saranam.

Devotees during this period wear blue, black or ochre (yellow-orange color) dress. Most devotees do not cut their hair or shave during the Mandalam period.

Now the devotees shout out the names of Lord Ayyappa and climbs the steep mountain and reaches in front of the glorious Sabarimala temple. Now they have to climb 18 golden steps to reach the chamber where the great idol of the lord is placed.

WHY 18 steps ?

18th chakra 

Is deep golden combined with prisma-colours (rainbow). 

It enables us to use our wisdom en power in service of the One. 

17th chakra 

Is platina and magenta in colour. This chakra will let us really feel the ONENESS with the Source, with All That Is, the worlds of flora and fauna and our deep connection with mankind. 

Chakra 16(Violet White)

 Fifth Dimension. Awareness of the multidimensionality of humanity. Healing the gap between what IS and what CAN BE. Seeing humanity as a whole, interdependent with all other lifeforms. Ability to affect the mass consciousness. The highest level of consciousness now available to humans.

Chakra 15 (Golden White)

 Activation of this chakra represents the end of individual perspective and the introduction of the collective power of similar energy fields merging together. It reminds us that the process of evolution is unlimited.

Chakra 14 (Deep Blue-Violet)

 When humanity moves beyond polarity, we will be able to use teh 14th chakra, which is the potential available to manifest in 4D. We must first master the integration of polarities. Through integrating the 4D chakra we are able to affect the energy grids of the Earth and other living things. At this time we can begin to activate this chakra, but full implementation is yet to come.

Chakra 13 - (Pale Violet-Pink)

The reality of 4D is abundance--inclusion--enough for all. Scarcity is the reality of 3D. Scarcity leads to fear and competition, the have and have-not mentality. At this time there is great resistance from the collective to let go of the vibration of scarcity, which hinders complete abundance manifestation. We can access this chakra, and help in the transition process.

The Twelve Major Chakras 

There are 12 traditional chakras plus the foot chakra all aligned (generally) in a row from above head to the base of the foot . Also the chakras pertrude from the body in three dimensions! If you were to look at them they would look like spinning vortexes. Most of the chakra also possess a front and a rear part of them.

Chakra 12 - Connection to the Great Central Sun (Monadic level of divinity), understanding of and communication with the universe, the unlocking of the abilities in the 11th chakra.

(Master Chakra) Found 12 inches above the crown, this chakra is the largest of all twelve chakras and when practiced, an individual can increase their ability to understand their relationship with the higher Light Dimensions. As this chakra is activated, visitations with the Archangels, Ascended Masters, and other Light Beings are available to experience.

Chakra 11 - Pathwork to the Soul, the individual's ability to acquire advanced spiritual skills (travel beyond the limits of time and space, teleprotation, bi-location, instantaneous precipitation of thoughts, telekinesis in some cases)

Chakra 10 - divine creativity, synchronicity of life; the merging of the masculine and feminine within, unlocking of skills contained in the ninth chakra

Chakra 9 - soul blueprint (the individual's total skills and abilities learned in all the life times)

Chakra 8 - energy center of divine love, of spiritual compassion and spiritual selflessness, your karmic residue, activates spiritual skills contained in the seventh chakra,

Chakra 7 ( The Crown Chakra) 

The Crown Chakra is located above the head. It has a total of 960 spokes and is the most magnificent of all the chakra to see when it is fully awakened. It is the only chakra that possesses a secondary central whirlpool. This chakra is predominately violet in color except for the central whirlpool which is similar to golden sunlight. This chakra serves as the spiritual connection for the individual. It is also the place where life-force enters into the chakra system. In an undeveloped state this chakra is the same size as the others. However as it grows and opens it will encompass the entire head. This encompassing of the entire head is where the idea of putting halo on various deities came from. It also is the place where the tradition of a king wearing a crown comes. The crown symbolizing the fully open and active crown chakra, indicating that the individual was ruling with God and not just ruling with the human self. A missaligned Crown Chakra can cause self centeredness, spiritual poverty and on a more physical side, poor reality control, and low energy all the time.

Chakra 6 (The Ajna Chakra) 

The Brow Chakra is located between the eyes. It has a total of 96 spokes. It is often seen as dark blue with or without violet and sometimes with pink-rose color coming from it. This chakra is the command center for your third eye. It contains your ability to see things beyond the normal physical senses, and as such is a source of intuition. This chakra is one of the two places where your spirit can leave your body in an out of body event or in an astral projection. A missaligned Brow Chakra can cause: memory problems, migraines, insomnia, neuralgia, high blood pressure, lumbago, sciatica, low immune system and acute sinitis.

Chakra 5 (The Throat Chakra) 

The Throat Chakra is located at the neck. It has a total of 16 spokes. Predominately blue but with alternating blue and green sections. This chakra is vital to communication, not only external but internal. It is the translator of internal feelings into external words and expression. A missaligned Throat Chakra can cause: persistent colds or flu that take a long time to leave, tonsillitis, acute infections, vocal and communication difficulties, and under-active thyroid. It also effects the sternum, throat, and endocrines.

Chakra 4 (The Heart Chakra) 

This chakra has 12 spokes and is golden in color but many times can dip in various shades from golden yellow to pink or light orange. The Heart Chakra is located near the heart. However this chakra can float and be off-center to the left or right of the hara line. The optimum position should be dead center the hara, in line with the other chakras. This chakra is very important because it is the only chakra that can transmute the high-vibration energy that comes from the upper three chakras into a form that can be used by the physical body and on the physical plane. Healers (of all forms) use this chakra to heal their patients, as it is the Heart Chakra that transmutes the illness into a more dense form, which then passes through the Foot Chakra and into the earth for processing. The term 'a broken heart' refers to this chakra breaking off at the stem that connects the front part of the chakra to the back. This break causes the heart chakra to shut down causing that empty feeling we are all very much aware of. A missaligned Heart Chakra can cause lung and heart problems (especially angina from over-exertion), asthma, epilepsy, fibrosis, chronic bronchitis, depression, midriff pain and claustrophobia.

Chakra 3 (The Solar Plexus) 

The Solar Plexus (also called the Navel) Chakra is located at the solar plexus (midway between your belly button and the bottom of your rib cage). This chakra is another one that has a tendency to float left or right, like the heart chakra. Again its optimum position is in line with the hara line and the other chakras. This chakra has 10 spokes and comes in various shads of red but also has random spokes that are green. This chakra is also one of the two places where your spirit can leave the body during an out-of-body event or an astral projection. It is the most sensitive of the chakras and the easiest chakra to send energy out of. A missaligned Solar Plexus Chakra can cause problems in the: adrenal, nerve tissue, skin disorders (jaundice), liver, spleen, pancreas and kidneys (re: elimination/cleansing), arthritis, hiatus hernia, diabetes, anorexia, body pH, and menopausal flushing.

Chakra 2 (The Spleen Chakra) 

The Sexual (also known as spleen) Chakra is located just above the sex organs. This chakra has 6 spokes each of which are a different color: red, orange, yellow, green. blue, violet. This chakra supplies some (but not all) the energy transferred during sex. A misaligned Sexual Chakra can cause: imbalances in the endocrine system, muscle tension, poor vitamin C absorption, thyroid problems, bed-wetting, nervous depression.

Chakra 1 (The Base Chakra) 

The Root (also called the Base) Chakra is located at the base of the spine. This chakra has 4 spokes red and dark-orange in hue. This chakra is also the home of the kundalini energy. The kundalini energy is the coiled mystical energy that rises upward through the spine energizing all the other chakras into full operation and potential. This chakra is also the chakra of your animal instincts and all those things that go along with survival. It is powerful (or it feels powerful) because the energy here is in raw form and is most near to a physical energy. A missaligned Root Chakra can cause things like: sexual or reproductive problems, circulation and blood deficiencies, constipation, bladder and Chronic Fatigue Syndrome.

The Foot Chakra

 

The Foot Chakra is often considered part of the Root Chakra. However in Hermetic philosophy we recognize this chakra as just as important as the rest. The Foot Chakra is located in the arch on each foot (note there is only one foot chakra, but it is located in the arches of each foot). This chakra (when it can be seen, for it is very difficult to make out) has three, two, or one spoke and changes in hue from a deep brown, deep green, or a pale yellow. This chakra is responsible for passing energy to and from your entire energy system into the earth. It is your grounding point where body energy is transformed into earth energy and the reverse. A good healthy foot chakra will help you to more easily manifest things physically, as it is the transformer of your thought energy into the earth system. A good Foot Chakra will also help you to stay physically focused and on task throughout your day. If you are clumsy or fall frequently it is a sign that this chakra is not working well. Foot massages can help greatly here as well as walking barefoot in the dirt or sand.

 After climbing the 18 steps they see the word “TATWAMASI”, inscribed in front of the Ayyappa Temple at Sabarimala, affirms that the Grand Reality of the Cosmos is same as the essential Divinity within us, and thus relates the devotee to the Lord Ayyappa. Now the devotee is God himself.