Have your generic background be layer 1, layer 2 will be the final video. In this video, it's probably just some generic circle wipe, but if you wanted to use another shape, place it over the Generic Background. Set the generic backgrounds track matte to "alpha inverted" and then use keyframe scale to 'wipe' the effect...0% to 200%(whatever you need)
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.