This is the yoga of Body Control and Breath Control.
One gets complete control of the nervous system, thereby every gland, organ and tissue in the body is brought under one's control. One gets complete control of the breath thereby bringing the entire mind under one's control.
The adept is considered as having mastered Hatha Yoga if he has complete control of the body through his nervous system and has complete control of the mind through the breath.
The advantages are good health, long life and the body is impervious to pain and the body is not distressed by harsh climates. One develops Iron body and Iron Will, the mind is not depressed and thoughts are under complete control. The body does not undergo Pain and the mind does not undergo Suffering. The twin weapons of karma are defeated.
The Hatha yogi can remain in his inward state unaffected by the changing temperatures outside, it can drop to subzero or it can go to blazing temperatures, he will remain unmoved.
But for novices bodily contortions are a means to put pressure on certain nerve points thereby increasing the energy flow to certain organs, this helps in defeating diseases especially life style disease. Also certain poses and certain postures will eject certain mental formations. It can trigger the mind to think in a certain way, it can create new mental habit patterns. Just as the mind has its effect on the body.
Similarly the body too has an effect on the mind, it can trigger a certain thinking pattern when it is held in a certain pose. Food, Bodily pose and breathing pattern all have an effect on the mind.
The method is to get into a pose and remain relaxed in that pose for a few minutes. This art was developed by studying healthy animals in their most relaxed states and their natural poses in that state. One also learns the effects of certain herbs on the body and mind.
Herbs can affect human consciousness and human body. Positive herbs that do good to body and mind are plucked during the waxing moon. Negative herbs those that do bad to the body and mind are plucked during the waning moon. This is used by Tantrics and those in black arts.
Traditionally, it is believed that Shiva gave this art to Gehranda and he in turn passed it on to Markendeyan. Gehranda is the father of Hatha yoga and not Patanjali and the most important text is Gehranda Samahita.
UG says one begins Yoga with the Shavasana. Unfortunately he is right.
The four steps in orderly succession are
Repose and relaxation
JK practised breathing and yoga exercises everyday for half an hour and he taught these exercises to his inner circle with a warning that now you will have some extra energy at your disposal, don't use it to create mischief.
Hatha yoga is a means to the end. The error is that people mistook this to be the end.