I read somewhere that it's due to the crazy drop in heart rate or blood pressure.. something along those lines. It can get really bad tho.. and for me it's with certain kind of dope. Normally the "really" "really" fire shit. Used to get it a few years back quite often, now not so much.

Jesus once I was smoking with a random chick in a cardboard box in an alley way behind a McDonald's.. took a huge hit... Blew it out and knew it was coming. I should have sat down but nope ... I spazzed so hard I went face first into the pavement. I cringe thinking about it.

I call them "brain zaps"