You don't "pass out", your in so much pleasure you can't keep your eyes open and they'll often roll into the back of your head, in a similar way to what a clean 'smacky' hit of MDMA can feel like, but obviously not the same (being a CNS depressant). Free of pain, free of anxiety, free of depression/sadness, you are in total bliss. Carefree and immersed in an amazing euphoria, no pain, be that mental or physical.
Personally, I LOVE nodding, to me nodding is not "passing out", it is the perfect balance between an incredibly blissful sleep, whilst being awake. You 'nod', as in you begin to drift off into a heavenly sleep, but you 'snap back' into being awake, then drift off again, and repeat. It may sound irritating, drifting off then, "coming too", over and over, but I assure you it isn't. It is what opiate/heroin addicts chase, that blissful nod.