The practice of Yoga Nidra is a practice of the opening of the inner chamber of the mind. It is not yogic sleep, it is not concentration.

Yoga Nidra will guide you to the hypnagogic state, an internal dimension of your consciousness between wakefulness and sleeping. Through the sound of my voice, you can access deep layers of your mind. Tell your mind to remain awake just by listening to the sound of my voice. 

I’ll be guiding you to move your awareness to various bodily sensations, emotions and images for the next 30 ms.