Masking the continous movement tutorial

Ok, so I set up the continuous movement tutorial for my own use, except I have vertical bars instead of circles. I have 3 layers. On bottom is a large graphic, the middle layer is another large graphic and the top layer is the “bars” w/ continuous movement.

Is it possible to use my bars as a mask, cutting through the middle layer and showing the bottom layer as it continually moves over the middle layer?

Just setting the “bars” layer as a mask to the middle layer doesn’t seem to work.

For a visual… http://www.manapedesign.com/clients/freemachine

Thanks in advance.