The Chewbacca occurs when you are having sex with a girl, and, in the heat of passion, you pull her arms out of their sockets.
"What the fuck, Jason, why are there two human arms in your bathtub?"
"Oh, I pulled The Chewbacca last night. Damn shame, third time this year. Sprinkle some more lye over those things while you're in there, by the way. "