In this video, Daniel Hinz covers track mattes and how to properly organize your layers so that they work correctly.
How to make a Track Matte. Track Mattes are useful for masking out moving layers. The first step is to use the pen tool to draw a shape over everything you want to keep. And take that shape layer and move it above the layer you're trying to mask out. So in this case, it would be the face layer. Boom.
Then go to the layer below that, make sure that you toggle the switches on. Go to Track Matte, and select Alpha Matte. Then, you want to make sure the shape layer is parented to the same layer as the layer below it. So in this case, it would be the body. And then you're done.