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Masking
From Chewiki Archive - YouChew:
1% Funny, 99% Hot Gas

Advanced masking is using masking/cropping tools to make the characters or objects do completely different things than what they were originally doing.
In Brief
What can advanced masking make characters do? Well, sex for one. They can also pop up in random places, fight each other, walk on different BG's, and many other things.