Realize to that not everyone can be hypnotized - I know I can't (though one therapist wished to convince me he feels otherwise - essentially to protect his reputation). I suspect that SPers fall heavily into this category, largely due to their excessive self-awareness.
As I understand it (from those few I've talked with who've been through it), if you can't account for a certain amount of time in the session, you were probably under hypnosis, at least in the classical sense.
These comments are purely anecdotal and my personal opinion and experience. While we're on the subject, what about EMDR (eye movement desensitization and reprocessing)? It's used mainly for PTSD (post traumatic stress disorder), but PTSD (imho) is often closely related to intense SP.