Quoted from RetroGamerJP:I've yet to see this, but I have seen guys take there hand and smack the side of the cabinet REALLY HARD in some sort of save effort. Not the side as in flipper button area but further up in the middle of the cabinet. I think it was Metallica they were playing.
This is just a nudging technique to get the ball off the orbit rail so it doesn't come down and hit the slingshots and go wild. You'll see it in tournaments when players are shooting orbit shots.