haha, my ass DOES stay on the bench! what's happening is my glutes are relaxed at the bottom of each rep, so when i flex them my hips come up and the Butt Surface Area (BSA) on the bench is lower. so while my hips are higher on the positive portion of the lift, my butt never loses contact. try it just in your chair. relax, then flex your ass. did you lift up a bit? same deal.
hopefully the judges won't penalize me for guys in front of the camera. i couldn't really "plan" for that and cheat when they're in the way, plus you can see the last 7-8 reps fully and my form hasn't degraded, you know?
no, my friend isn't a poster here, but he wanted to try it out anyway just to see if he could keep up. did pretty well i thought.