Using a flv as a mask...?

I’m trying to replicate this effect on …

http://www.tedperez.com/archive/sweeneytodd/

The way the image on the home page is revealed…as if a gradient is bleeding into a piece of paper. It was done using an animated smokey gradient type effect made in after effects, and then exported as a swf.

I actually have the swf file. The issue is, how do I use it to mask out an image.

So basically, i have a swf file with an flv in it, that is using it’s alpha…and I want it to mask another image in flash.

how would i do this?

How did they do it?

Thanks,