Hi Cody,
When you apply a matte material onto an object, it doesn’t quite make the object transparent, but rather it’s as if you applied the background color/texture onto the object. So, in your scenario, if you apply the Matte material onto an object, it can mask out an object that is hiding behind it. As you move the object with the matte away, it will uncover the video for you.
Part of the confusion too is that the Matte coloring doesn’t show up in the ProAnimator interface unless you do a test render frame, or the real render. You will just see the object with a gray material applied. To do a test render, hold the control (command on Mac) key, and the “Render Movie” button will become a “Test” button. When you use a test render frame, you can see how the matte coloring works.
Hope this helps you out.
Best,
Edward