During yoga nidra you remain in the threshold between waking and sleep while following verbal guidance through body scan, breath awareness, and visualisation. This state is associated with delta brainwaves and significant drops in cortisol. A 30-minute session is often described as deeply restorative.
Yoga nidra is particularly useful for people who find sitting meditation difficult. Because you are lying down with eyes closed, there is no posture to maintain. The instruction does the work of anchoring attention. Free recordings are available through many yoga teachers and apps.