I used to do this with a friend of mine when we were teens. We would take a towel and roll it up length wise. I'd stand behind him and he would take some deep breaths and hold it in. I'd place the towel around his neck and pull tightly. As soon as he would collapse, I would remove the towel.
Sometimes he would start convulsing, and other times he would wake up without convulsing. He never felt it when he did.
I never felt it when I started convulsing, and he said one time I was convulsing so bad he thought I was dying.
My guess is when you stop the blood flow to the brain, and then the flow starts going again, it causes your body to react by convulsing, or it's your body struggling to get air back into you. I don't know, I'm not a doctor!