According to Wikipedia: Yoga nidra (Sanskrit: योग निद्रा) or yogic sleep is a state of consciousness between waking and sleeping, like the "going-to-sleep" stage. It is a state in which the body is completely relaxed, and the practitioner becomes systematically and increasingly aware of the inner world by following a set of verbal instructions. This state of consciousness (yoga nidra) is different from meditation in which concentration on a single focus is required. In yoga nidra the practitioner remains in a state of light pratyahara with four of his senses internalised, that is, withdrawn, and only the hearing still connects to the instructions. The yogic goal of both paths, deep relaxation (yoga nidra) and meditation are the same, a state called samadhi.
That sounds a lot like a freedive to me, apart from in a dive we have even more complete pratyahara (withdrawl of the senses) with only partial sight occasionally necessary for reference on the line - all other senses are ideally turned completely inwards.
In my Energy Beyond Breath course, I have created a Yoga Nidra designed especially for freedivers. It guides you through a dive and the inner world of consciousness that we ideally want to cultivate during our dives. This is the key to maximising our energy and remaining fully in the moment. Only then can we pull off the perfect, easy, blissful dive.
Energy Beyond Breath is the third in my Yoga for Freediving online training courses. If you haven't experienced them yet, you might want to check out Deep Relaxation and Manage Your Mind first before you get started with this latest in the series. They're the perfect way to complement the hard work you're already doing in the water, and ideal for keeping you motivated and growing over winter!
Purchase your own copy of Energy Beyond Breath, to get the full online course