You can access 7,20 through either "hole" in the top wall of 7,21. The right hole gets you the star in 7,20 and the left hole helps you get to the star in 7,21.
If the timing is right (which is why MetaMetaPuzzle said you have to "waste" a move), you can push a dead cat through the splitter that would otherwise destroy it. If you've reached 12,13, the mechanic is demonstrated there.