Corpse Pose - Shavasana

This posture is frequently done at the end of a yoga practice, bringing the body into complete rest for 8-15 minutes while the body integrates the benefits of the practice. But true Yoga Nidra, or "yogic sleep" is achieved by taking more time to systematically scan the body from one end to the other, relaxing the muscles, bones, organs, and calming the mind. This relaxation, with practice, becomes so complete, that

although the mind does not lose consciousness or actually fall asleep, there is no sense of the body, and the mind is in a calm, dreamy state of no coherant thought.
A teacher can guide her students into this state by talking them gently and gradually through the body scan, suggesting they allow their mind to rest in the exhalations, feeling the body grow heavy, all sharp edges softening, etc. Some gentle meditative music, nothing with a tune or beat, helps.
There are CDs available to talk you into Yoga Nidra, which are nice to use in your own practice, and to give you ideas for teaching it.
15-20 minutes of Yoga Nidra is as refreshing as an hour nap. The brain waves are altered and the mind becomes receptive to positive affirmations; the immune system is free of stress, and the whole body feels cleansed and refreshed upon coming back. Slowly guide your students back by having them take a few deeper breaths, moving fingers and toes, rolling head side to side, etc. Then have them roll to one side and gently come back to a seated position.