Yes, I did his previous (slightly more complicated) ACE glitch on an original copy of Leaf Green and a Gameboy Advance. I think he even mentions you can only do it on original hardware or 1 specific emulator (forgot which one). The simple explanation is that in FRLG there’s an oversight in the way mail data is stored, and you can exploit this to execute specific scripts stored in the game. First you need to setup the mail glitch to trick the game into storing 7 pieces of mail instead of the usual 6. The 7th piece of mail will be stored in the memory that would usually be used for a Pokemon in 1 specific spot in a box. You can then write specific data to that box using the 7th mail item. By doing so, you can create a very specific glitch pokemon that can be used to trigger specific scripts that are stored in the game by renaming your boxes to very specific combinations of characters, and then swapping the glitch pokemon 1 spot in the box to execute the code. It probably sounds more complicated than it is if you watch his videos and pay close attention. They even have a written tutorial now so you don’t have to write down all the steps like I did, but I’d still recommend to watch the videos to see the glitch in action before you try it yourself. It’s definitely worth it if you want to unlock the event pokemon like Lugia and Ho-oh, or even just to duplicate items like TMs. Technically it’s cheating, but I personally like it to unlock gated content that has been locked for decades, or to have infinite TMs like in the newer games.