I've seen that, but I just don't get how to apply it to what I want =\
This is about as close as I can get: Example
As you can see it's pretty much the opposite of what I want!
Except I want the red square to be completely invisible, the mask invisible as well. But when the mask goes over the red square you can see it through the mask.
Feel like I'm close but I'm not sure what else to try.