This article contains spoilers for "Star Wars" #13.
That's a Wookiee pulling an arm out of its socket -- specifically the arm of a protocol droid. Aaron and Deodato weren't content to keep the callback to just that one page, though. When the story returns to Chewie's attack, he's doing something even more awesome than ripping an arm out of a socket.
Turns out Han Solo doesn't exaggerate when it comes to a Wookiee's temper. "Star Wars" #13, part three of the "Vader Down" storyline, is on sale now.