Backdrop Effects is a Mac OS X 10.5 Leopard iChat feature that “removes” the background of your video chat and replaces it with a picture of your choosing, leaving you in the foreground. It works much like a “green screen” used by actors in a Hollywood studio.
This works by taking a snapshot of the background when you are not chatting. Since the computer knows what the background is, parts of it can be replaced with images or movies. Since you were not part of the background when the snapshot happened, you will not be ignored, hence you or any other object will be able to move “on top of” the background.
The feature is present in Photo Booth, too, as shown below: