hi boa, i have something similar that happns.
if i sleep laying on my back, i get to a point where i freeze & cannot move, i am in what is best described as a semi conscious trance, but not in control, as though my thoughts are active but my contol over my body is severed, sometimes i feel as if someone is on top of me, sometimes it's other things, like screams, i almost always hear screaming, both in my ears and in my head.
most of these these are i believe symptoms of sleep paralysis, (something i first heard about on this board) sometimes these things also happen if i am sitting relaxed and in a non defensive position, usually if my head is rolled back i am more open to it, i believe it happens right at the point between being awake and being asleep, where normally you would just switch off into unconsciousness, but for some reason you again become conscious of your surroundings when your body has gone to sleep... this being said, that's not always the case because sometimes i am deep asleep and it wakes me.
for what i have, it is easy to induce because it has happened since i was a kid so is 'normal', but to a large extent i think it fits the bill of sleep paralysis, with maybe a few optional extras.
not sure if it's the same as what you're talking about but there are similarities.