The best way to understand what Yoga Nidra is to understand who is practicing it.
The one who practices Yoga Nidra is a yogi and the clearest definition of what a yogi is comes from the Bagavad Gita 4:18 as follows: “One who sees inaction in action, and action in inaction, is intelligent among men, he is a yogi and has accomplished everything (he is in the transcendental position although engaged in all sorts of activities).”
This definition evokes the obvious questions, “What does inaction in action and action in inaction mean?”
Inaction in action and action in inaction means that your very being is doing . For example in deep sleep you are not consciously active yet there is no discontinuity to life.
The practice of Yoga Nidra approximates deep sleep. In the practice ‘you’ are inactive yet you are awake, alert and active as your true nature – thus you experientially understand ‘inaction in action’, ‘action in inaction’ and you also then know what it means to be a yogi who has accomplished everything.
In this light Yoga Nidra is the practice of inaction is action and action in inaction.