I'm trying to get a clip in the background of another clip while also have objects move in front of the background clip. I was wondering how is this done. I have already tried using a composite layer such as multiply and it doesn't give me the effect I'm looking for.
http://youtu.be/Fvs6YaPMQ4A?t=3m31s from this clip around 3:31-3:38 is somewhat what I"m looking for with background images going on in a foreground clip. I"m not sure if this is an image mask or what exactly.
I use Final Cut Express HD on a mac
Any help at all would be greatly appreciated. thank you.

