Cool masking trick anyone?

So I’ve seen this masking trick from time to time and I’m trying to figure out how to do it. It is usually done when there is a banner and the pictures in the banner are revealed by what almost looks like a falling domino effect. It usually starts from the top left corner and then fills out to the bottom right corner. It’s revealed usually by what appears to be a bunch of small squares. Has anyone ever done this before? If so, can you give me a tute or some pointers? Thanks!

JP