Animation Special Effects: Where can I find Tutorials on this?

I would like to find tutorials that teach anination special effects:

For example, You have two layers: The bottom layer has an image and the top layer has a black rectangle. Or what ever. And you want to have a moving anination of a brush or erraser removing across this top layer revealing the image on the bottom layer as this top layer is in motion.  And it is not frame by frame but more like a motion tween. Its fluid movement.

Like a brush stroke moving across the page revealing the image below.

Do you know where I can learn how to do these kinds of special effects?