Reveal entire layer in after effects with mask partial overlap
So, if we will continue analogy with Lego blocks:


First you need add On &amp; Off effect for your blocks. You can find here how.
Then you shall add Collision detection. You can find about it here.
Now you need to trigger On &amp; Off effect when your object (mask?) collide with block. 


I never tried it myself, but now really want to test it!