By 'choke' I am assuming that you are alluding to Matte Choker or Simple Choker. These require a non-full screen alpha channel. If your image doesn't have an alpha channel, then you can create one using Shift Channels. I suggest that you search the COW for previous posts on how to create mattes from one of the color channels.
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Well choke seemed like the best way to describe what I'm trying to do. It may not be right. If you can picture the image I posted as the full silhouette of a man w/out any fx. Then it transforms into that pic, which looks like the edges moving inward to me. If that's not what it is let me know what it is.
**I look down at where you're standing
Flock of sheep all on display
With all your lies piled up around you
I can take it all away**