GameCube Scrubbing

What is GameCube scrubbing, and what does it do? All GameCube discs are the same size but games rarely use the whole disk and the rest is filler. Let's use Mario Party 7 as an example. It uses about half the disk which looks something like this:
General disk outline of Mario Party 7
Since only half the disk's used it should zip really small right? Well, it gets a bit smaller, but not as much as you'd think...
We zipped the game but the filler is huge. What gives?
So the game shrunk but the filler didn't. Why's that? GameCube discs are a bit quirky and their filler isn't actually empty. Instead it's filled with random data, and random stuff just doesn't compress. This data's completely useless, so what if we replaced it with something zip-friendly? Well, that's called scrubbing! We simply replace the useless filler data with zeros and it compresses insanely well.
First we scrub the filler, then we zip it.
Now the filler's tiny!
With the filler scrubbed our zip file is much smaller, shrinking from 1.18 GB to only 394 MB. That's three times smaller! And the best part is, it works exactly as the original.