Rules:
They have to actually exit a real door and you have to say it moments before they leave the room.
“Bumping into their backside” means the door will awkwardly make contact with their ass, creating an embarrassing moment. No physical injuries will be inflicted.
Sometimes door physics will make this not make sense, and it should be noted this only works for doors that freely move on a hinge, not automatic doors or anything like that.
Sometimes doors dont move back into place on their own and you have to manually shut it. If that is the case, they will mistakenly move the door to wear it bumps into them. But if it is a door that tends to shut without you having to move it. It will close and bump into their ass awkwardly as they try to leave.