The act of going out with one roommate, then realizing you like the other one more and wanting to switch. However, if you simply break up with your girlfriend her roommate will be too loyal to her to go out with you. So you request a menage a trois to your girlfriend, and she will be so offended by the fact that you're attracted to her roommate that she will break up with you. Then she will tell her roommate, who will understand that you like her. Your ex will not be against this in any way, because she was the one who dumped you. You will have successfully performed the roommate switch.
Made popular by Seinfeld.
Jerry: I want to go out with his girlfriend's roommate, but I can't think of a way.
George: You should try the roommate switch.