I have a short 3d video with a distinct alpha black/white pass mark, how do I use the alpha as a mask pass?

I have a short 3d video with a distinct alpha black/white pass mark, how do I use the alpha as a mask pass? I want to add a distortion of the heat wave to the entire video, but not on the alpha pass area. How do I configure this?

Matte:

http://helpx.Adobe.com/after-effects/using/alpha-channels-masks-mattes.html#track_mattes_a nd_traveling_mattes

Start here learn After Effects:

http://Adobe.LY/AE_basics

Tags: After Effects

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