"Jowling," (sometimes known as "slap n' flap") is a little hard to capture well, but always fun to view.
How do you do it? Get your subject to relax their facial muscles completely (to the point that they have no facial expression). Then, while staying relaxed, have them open their eyes and mouth (and try to keep them open) and shake their head from side to side as fast as they can. Start snapping (use a flash and fast shutter speed to freeze the 'action')!