There are really other ways to trigger this exploit, but i will explain one of the ways that are common and are similar to other ones. 1: go into the prison corner near the window thats closest to the room you cant normally enter(you can use other corners too). 2 take your gun out and point the gun directly up facing your back to the walls than holding "ALT key" and moving backwards for a bit than stop then keep going then stop until you have gone inside. once your inside you can go through walls and see through them too, sometimes you have to crouch and stand up to go through walls or face your back towards the wall you want to go through.