I am still a practising student of Yoga and feel unfit to speak some of these things but for the sake of completion I am posting this.
Yoga is, according to Patanjali, "ChittoVritti Nirodhah", meaning cessation of mind modifications - mental tossing. But one can not control the mind, if one has not mastered control over his body! So this is very important that one has control over body as well, and for that a healthy body is very essential.
Once someone has learnt to control his mind, he masters control over his involuntary and autonomous metabolic functions! But before that, one has to learn to control his voluntary system!
Imagine, you can command yourself to sleep, to excrete, to urinate, to pass gas, to lower Blood Pressure, to get rid of your headaches and migraines! Why it is important to pass gas, excrete and urinate at will - atleast to some effect! Any waste product should not be retained for long in the body as it acts like toxins to the body, causing harmful effects.
In fact modern medicine and science is slowly realizing that the ancients wisdom of Yoga has direct medical benefits based on scientific principles. But not all is yet realized by them, as Yoga was a lost science till Dr. Sivananda revived it and made it famous throughout the world 60 years back! Still I feel that not many people have yet realized its powers, including so called many Yogis. This is because there is so much you can do and learn based on your personal experimentation!
One can cure diseases like Insomnia, Apnea, Blood Pressure and even some immune disorders, as I have cured all of them to most extent- though I suffered the Sleep Disorders for last 35 years, Immune Disorder for last 11 years, and Blood Pressure hitting 215/115 and not coming in control with 3 medications! Most of the diseases are present in the body because the natural cycle and the balance of the microcosm system in the body has gone awry. This can happen because of the imbalance in environment we live in, improper balanced food we eat, too little or too much sleep, emotional instability, mental stress and agitation, spiritual disconnect and lack of adequate exercise.
So the eight steps are for some very important reasons, are spelled out by Patanjali in his Yoga Sutra. Without a character and an ethical and moral framework, one can not achieve the maximum benefits of Yoga.
1. Yama - Restraints
a. Ahimsa - Non Violence and Non Injury at Physical, Mental, Emotional level. Hear nothing Negative, say nothing Negative, and speak nothing Negative. Negative energy inside and outside is as harmful as sitting in a radioactive room! Yes it is easier said than done, but one must always strive! I am guilty of it and not feel justified in mentioning it. But over years, I see this makes you a better person, more calmer, more at peace and more happier and contended.
b. Satya - Truth-full-ness, Non Telling of Lies - Science has shown that the brain has to work harder when one lies! You burn your fuel and pollute your system twice than what is required!!Yes the modern society is not conducive to this value system and encourages you to lie. So try to find your balance!
c. Brahmacharya - Chastity, Sublimation of the Sexual Energy. Like Hunger, Thirst, Mind Oscillations, one has a tough time controlling sensual desires, specially when a provocation happens at sensory levels. The idea here is to keep this to minimal and not be crazy, possessive or carried in sway about it.
d. Asteya - Non Stealing, Non Covetousness, Lack of Jealousy. We do not want out conscience to be haunting us - that is if you have a conscience! Some of us may have a conscience surrounded and built on false values, result in all justifications!!
e. Aparigraha - Non accepting of gifts or bribes in exchange of expected favour or services to be returned. Basically this reduces anxiety and expectations - like one doing one's duty without the ego of "I", without attachment to the outcome and having expectations, and as a service to the Divine.
2. Niyama - Observations
a. Saucha - Purity Internal and External, which is part of body hygiene, environmental cleanness and lack of pollution, and purity of mind.
b. Santosha - Contentment. If we are not contended, we are unhappy and angry, which is not good for the body and mind. We get tied to a very narrow perspective and forget that all that is here, good or bad, shall also pass! There is nothing permanent and at end everyone dies here and leaves! That is why identification with the Soul or First Principle, and not with Body, Mind, Intellect and Ego helps, and one sees the Eternity in the Self. Please see my posting on Atom and Atma.
c. Tapas - Austerity. We should not ask a lot for ourselves from the environment and others. If everyone says, "I want more and more", we can see the limited resources of the nature being challenged and destroyed, and conflicts arising. Man is the biggest beast who has killed most of the animals on the planet and ruining the planet earth for his selfish gains. It looks like we are on an inevitable collision course with the inevitable - that I am afraid is going to be unthinkable and indigestible for most of us - as we are ruled by the same power hungry people and nations, who want "more and more".
d. Swaadhyaa - Self Study on Scriptural Knowledge. This does include books of knowledge in any field that benefits mankind, the Nature and the Self.
e. Ishwara-Pra-Ni-Dhaana - Surrender to the Divine. In Gita it says that when one is equipoise in heat and cold, in happiness and sorrow, among foes and friends, and in fortunes and misfortunes is a realized person. This comes when one stops identifying with the Body, Mind, Intellect and Ego and identifies himself with the Divine Principle - the Self or the Brahman. Become like "a leaf in the wind" or "a fish in the universal aquarium", where it does not matter where you are, as you always part and parcel of the Divine Principle in whatever form you are!! Please see my posting on Atom and Atma.
3. Pranayama
Control of Vital Life Energy through different breathing exercises mentioned in Yoga. This is a field by itself and one will learn new things as one practices this over time. The Anatomy Of Metabolic Respiration, The Anatomy Of Mechnical Respiration, and Kapal Bhati Steps And Its Medical Benefits.
4. Aasana
Different postures and locks which create pressure points, stretch and massage different parts of the body. Pressure points help stimulate nervous plexus centers, or chakras and also has a "squeezing effect" on different parts of the body in disposing waste deposits, speeding up excretion of gastric juices, harmones, and neuro transmitters. Stretching helps blood rush in and increasing blood flow, hence improving cellular metabolism and also exercising the cardio-vascular and the respiration system. Please see my post on Anatomy Of Metabolic Respiration.
5. Pratyahara - Withdrawl
This is for the spiritual seekers who want to experience the Essential Truth - The First Principle, that goes by many names, The Divinity, God, Brahman, Shiva, etc. What it means is that one has to evolute by involution as part of his evolutionary growth, by involution of his mind from the sensory world to the inner world of the soul. In order to conquest the Senses, Body, Mind, Intellect and Ego and feel eternally Peaceful, Happy and Blissfull, one needs to withdraw till one masters control over Mind, and then can still operate in the Material World. It is also called Uparatii.
6. Dharana - Concentration
This is single minded focus on any material, mental, emotional or spiritual object. It could be Sound, Picture, Smell, Some Positive Thought, Longing for Lord or Soul, Mantra, AUM, or any spiritual object of admiration, like Cross, Star of David, etc. In Tantra, Yantra, Tantra, Mantra, and Mandala are employed.
Breathing plays a very important part in this step. Sometimes the intensity of the mind is so strong in positive direction of achieving this goal, that it automatically happens. Sometimes, one has to use Pranayama to control the mind, and transcend the mind. It is very rare and difficult to transcend the mind without breath control unless one has such a wonderful longing and desire of the object of focus that it happens by itself.
All creative efforts, like Painters, Singers, Dancers, Thinkers, Scientists, Philosophers get this moment unconsciously in their "Ah" moment. Among the spiritual seekers, this is called the Rapture moment, where one experiences
7. Dhyana - Meditation
This is a very advance stage where one has transcended the mind, has perfect control over it, is beyond sorrow and happiness, is equipoise among friends and foes, takes success and failures, pleasant and unpleasant with same positive attitude. He is in constant remembrance of the First Principle - The Lord, and it need not be with eyes closed, though very difficult.
One can do open eye meditation by looking at presence of the same First Principle in all the forms and shapes, and animate or inanimate. Some people, can tune in to their breath, and feel the breath come in and go - and do a silent chant mentally saying "So Ham" - "I Am That", the First Principle. Note that when you breath in, the sound "So" is naturally made, and when you breath out, the sound "Ham" is naturally made.
People who have reached the Kevla Kumbha stage, which itself has many degrees of advancement, they can constantly retain and hold their breath - it does not mean that they are not taking oxygen, but they are taking minimally that comes in contact with the nostrils, without the diaphragm, chest, collar, throat or nostrils doing any inhalation or exhalation. In a very advance stage, one may just sense the subtle current of the air, like a whirl-pool of inward and outward motions, more like circulatory ones, at the tip of the nose, or in middle of the nose or in the upper nose, near the third eye. Some practitioners may develop some flap which would open and close when a "molecule" of air is allowed to flow in beyond in their meditation.
8. Samadhi
This is for the Spiritual Seekers who believe in the constant and repeating cycle of Death and Dirth and want to get liberation from this cycle. This is called "Moksha".
This seems to be Vikalpa and Nirvikalpa Samadhi. The earlier one is with the thought and the later one is without the thought. Vikalpa Samadhi is also called Sahita Samadhi - or added Samadhi.
This stage is all text book knowledge to me and I do not know anything about this stage.
While the sequence of the steps are important, it does not mean that one has to get a 100% mastery in one step and then go the next step.
There is nothing like a "black" and "white" demarcation, and this journey is iterative and running along many parallel tracks.