I tried doing this with a composite shot containing the matte:
The idea is that the text, when moved past the black, will go invisible:
However, as I set the text layer's matte to the composite shot containing it, some weird thing happens. 1) It does not go invisible, and 2) the area containing the white shows. And in black:
Just incase, I intended for the matte not to show at all, of course, so I have its visibility toggled. I have the Set Matte settings to:
Matte Source: Luminance
Am I missing something?