Whereas a normal woman's vagina can only take so much punishment before becoming sore, a woman with a "double-jointed" vagina can have sex with multiple partners (4+) throughout the day with no ill effects.
Person 1: Where's Carrie?
Person 2: She is still in her room with her third guy of the day. Number four is on his way in an hour or so.
Person 1: Oh my gosh, Carrie has such a double-jointed vagina!
Person 2: I know, right?!