Animated Animation Masks

I have the “blender book,” but it doesn’t quite explain very much about animation masks. It is just part of a massive sequence editor tutorial. Is it possible to add an animated animation mask? And if so, how?

I am trying to make a scene composited with live video, and I need the 3D objects to go behind the subject in the video. Is there any other work-around besides animation masks?

try looking for the compositing tutorial at