From my experience they are just standard metal chairs you can get at the local store. I've never heard of the back/middle being popped out or anything like that - I'm unsure what that would achieve other than make it flimsy as fuck. The idea is to make it look like it hits when in reality you try to get a hand up (obviously). Remembering that many chairs that have been used have had announcers/time-keepers etc thrown out of them so they must be able to hold the weight on a grown person.
The last thing I'd want is a flimsy thing where the middle can fly off and take off half my scalp tbh!