This is called "Jowling." I saw this on the digital Photography School blog the other day and it looked too fun not to try out. However, a self-portrait at eight in the morning, at work, with a headache, it proved much more difficult than previously indicated.
As you can see, I just end up looking like Cletus The Slack-Jawed Yokel. Not attractive.
The idea is that you relax all of the muscles in your face so everything just hangs and then shake your head back and forth as fast as you can while someone takes a picture - with a flash, of course - trying to catch the motion. I am sure this would be much easier after a drink or four to loosen up the muscles a bit.
I'm going to see if I can find a few victims, er, volunteers, tonight and we'll see what happens.
(jowlers.com via dps)