'bouncy' buttons

if you go to http://www.blookie.com/blookie/3rdHand/index.html and look at the buttons that slide down at the top you will see what I will be talking about.

Here are buttons or mc acting like buttons that, once the mouse is over it, ‘springs’ into a deformed x and y. the x and y deformers go to a certain size, then a bit over that then a bit under that then over and under and so on so it looks like it is springy.

Does anyone know how to do this?

Thanks.
Van