That such suggestions actually work may well be a myth. On the other hand, the idea that this suggestion is often given is not. I have no experience with live hypnosis, but I have, of my curiosity, found various (purported to be) hypnosis induction soundtracks on the web, and "when you awake you will not remember this but the suggestions will continue to work" was a frequent theme. Perhaps this is life imitating the myth.