This picture on The Assute Bloggers site is no longer attached to this article at the New York Daily News (it had been). Take a close look at the picture. While shaking hands Biden has grabbed Palin’s elbow with his left hand. This is a control maneuver. There are several aggressive moves that can be executed from this position.
It is just some interesting body language that indicates an unconscious level of hostility on Biden’s part. Mr. Fu is not sure that Biden is capable of effective physical violence, but the energy in the shot is something to ponder.