Well, what the subject said.
How do they work?
I managed to achieve something like an HFO while out of trance. I was sitting upright with my eyes open. I needed to excite myself by… adoring, basically, visual stimuli instead of just looking at them, by being surprised by things I had already seen. So I suppose it may have been a form of self-hypnosis, in that I left my normal state of mind. I was also moving some muscle that felt as if it was behind my perineum, but I'm not sure where to draw the line between the physical and mental stimulation. It was mild compared to the hour long, flat-on-my-back hypnosis sessions.
Another type of HFO I've had - my first, actually - was a rush of euphoria after about an hour of altered breathing in a trance state. This was to the Bambi files. I think I had two, actually, one definitely in the 9th file. I remember my cock twitching every time I was called a good girl. (This may have been conscious at first.) However, after the session ended, I noticed most of my body was tingling and slightly numb, even my ears. And of course my brain was still swimming in endorphins.
I think most of the euphoria came from a state of mild hypoxia (oxygen deprivation) from breathing in through my nose and out through my mouth. It's not quite auto-erotic asphyxiation, but why wouldn't a mild oxygen deficit have a similar effect?