To get the picture (pun not intended) of what I'm talking about, do this: open two Finder windows, make one larger than the other, and put the smaller one over the larger. Now, fire up our little camera friend, and tell it to take a snapshot of the larger window beneath. Most people would expect the smaller window to show up in the picture, but surprisingly it removes anything obstructing the selected object off of the final shot.
Also try it with other UI elements, like dock icons and the Exposé blue blob! You'll find out it not only removes objects over the selected target, but also takes out anything behind them!

