Sweeney Todd flash effect...?

If you go into the site… there is a mask effect that is used to reveal the image on the home page. It sort of bleeds outward in a very organic way.

http://www.sweeneytoddmovie.com/

The file they are using is an flv…that looks like this…

http://sashagrey.com/burn.html

But how can you mask another image using an flv with an alpha channel? did they somehow use blending modes? or am I missing something about masking?

Thanks.