You take a girl (or rarely, a boy) whom you have no respect for into the room and commence having sex with her/him.
Right at the moment of climax, you give the prearranged signal and all of your friends emmerge laughing to scare and severly embarrass the victim
No what happened?
At that party last weekend Cole Closet Monstered her! Matt, Jordan, and Evan were all hiding in the room
guy 2: "yeah it's about time, that fag was such a closet monster."