I could be wrong, but my best guess is it's Compiz stuff. If I remember correctly, adding Compiz to your desktop (all the transparency/3d/jiggly stuff) lets you press a button to make all your windows "smaller" to your desktop and click one to enlarge it. Kind of like the ALT+TAB window chooser, but in 2d rather than 1d (series of apps instead of a row-list of them.
Though as I said, I could be all wrong
