How do I achieve this masking effect?

Hey All,

I’ve seen stuff like this before and have always wanted to know how to do it… I know he uses the shape tween instead of motion, but I can’t seem to replicate it… can anyone help me?

Look in the main window (the yellow one) the way the mask kinda drapes down revealing the content… http://www.clgd.net/

Thanks in advance!

Ben