Excerpted from the Yoga Nidra article at SwamiJ.com:
YOGA NIDRA IS NOT AFFIRMATIONS OR AUTOSUGGESTION
Yoga Nidra is not a practice of autosuggestion or verbal
affirmations, whether affirming desired personal qualities or worldly
objects that are desired. It is also not a process of verbally
affirming that you are in Yoga Nidra. If you are still in such a
conscious level that you are still making oral affirmations of any
kind, then you are definitely not in the subtle state of Yoga Nidra.
In such a case, the conscious, Waking state mind has not yet receded
into the cause from which it arose.
Yoga Nidra means Yogic Sleep. It is a state of conscious Deep Sleep.
In Meditation, you remain in the Waking state of consciousness, and
gently focus the mind, while allowing thought patterns, emotions,
sensations, and images to arise and go on. However, in Yoga Nidra,
you leave the Waking state, go past the Dreaming state, and go to
Deep Sleep, yet remain awake. While Yoga Nidra is a state that is
very relaxing, it is also used by Yogis to purify the Samskaras, the
deep impressions that are the driving force behind Karma.