This is crucial to yoga and can actually be quite hard to master. In letting go into a deep state of relaxation one allows the body and mind and subtle energy to integrate and all kinds of subtle benefits accrue. Avoid drafts and hard floors and use a blanket for warmth if necessary. You can do this exercise in bed before sleeping too as well as after postures.
The method is simple. Allow at least 10 minutes. Slowly lean back from a sitting position with legs outstretched and feet a foot or so apart and let the spine stretch out on, or close to, the floor, vertebra by vertebra, until the head is resting on the floor. Let the hands go loose a few inches from the sides of the body, palms facing the ceiling. Let the feet go loose too, and gently tuck the chin in to the chest.
Consciously set a time for the period of relaxation in one's mind - this tends to be quite reliable as an inbuilt alarm clock. Take three long deep breaths through the nose and release them through the mouth while letting go of the mind and body.
Then gradually work up from the soles of the feet to the crown of the head relaxing the body and mind and becoming aware of any sensations of energy or well-being.
Gradually one should begin to lose interest in the senses and the brain will become stiller and one may fall asleep, but instead one should try to stay gently aware until one slips into an inner world in which a sense of deep wholeness may arise without one even noticing. Sometimes a sense of light may arise too. This is entirely natural and should be enjoyed.
As one comes back from this state into the world of the senses be gentle
with oneself. Take a few deep breaths, acknowledge the exercise is over
and gently get up. It may help to avoid any activities for a few minutes.
Allow half-an-hour before eating or drinking.
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