The -20 trick is a method of changing a regular color code for Super Mario 64 to make it work in hacks like Super Mario Star Road. Unlike the previously known trick, the ADMA trick, the method copies the colors exactly and the code functions exactly like Super Mario 64, just with some minor differences, and is much easier to use compared to said trick as well.
Basically, it requires one to subtract 2 from every 7th number, unless it is replaced by a letter (A -> 8, B -> 9). In MM54321's videos, the trick was introduced by MarioGame2222 in Into the Fourth Dimension.