I experienced it for the first time in self bondage. I cant remember anything. I remember feeling lost in complete ecstasy and euphoria and my breathing slowing. And then I remember being propped up on my elbow with the back of my hand on my forehead breathing hard trying to think. In between I have absolutely no idea what happened. I only have a flash of feeling like I was falling deeply asleep.
So that made me wonder, what the hell happens in the brain that causes this to happen? Being the nerd I am I turned to my faithful google.
As far as I can tell, states of consciousness can be affected by sensory overload. As in, you are feeling so much that your brain cant handle it. Sensory overload is connected to autism, Ashberger's, and it is even used in prisoner torture (not BDSM like real torture) but as far as I can tell it can happen to anyone. And its harmless.
I found this which describes my experience verbatim:
Situational factors affecting our states of consciousness:
1. Reduction of sensory information from external sources; sensory deprivation (isolation, solitary confinement, extreme boredom)
2. Increased sensory information; sensory overload leads to profound emotional and mental arousal.
3. Hyperalertness or mental involvement
4. Hypoalertness or a relaxation of mental faculties; achieving a passive state of mind.
5. Alterations in body chemistry
Commonalities in altered states:
1) alterations in thinking
2) disturbed sense of time passing
3) change in emotional expression
4) perceptual distortions)
5) change in meaning or significance
Personally I found the experience to be unbelievably amazing - I described it to my friend as dying, going to heaven, and coming back. I wouldnt be surprised if that is what it feels like when you die and go to heaven. But it was just way too intense. I will not be doing that again anytime soon.
I even read something that said this is sometime assumed to be alien abduction.