I bet he has access to many EEGs. This method works by dissociating REM sleep. Again I'm playing with this since 1997.

I cant' remember which paper was but I've read that sudden audio clues trigger PGO activity. May be one from Tore Nielsen. He has some good articles on sleep mentation and sleep onset that you should check out.

The PGO theory seems plausible for me since I often feel movement in my ear (middle ear muscles) and a blow in my solar plexus (don't ask) when disturbed in clear sleep.

There is no need to make this a science, just use it.