It's in the same thread that updates without burning fuses: https://gbatemp.net/threads/how-to-...nofficially-without-burning-any-fuses.507461/
In a nutshell, it breaks the cartridge slot completely to prevent it from being updated so you cannot play cartridges on any firmware as long as you have the fix. Ideally, remove it when you return to a lower firmware.
Thanks, ill do a bit of digging there later!!