|Please note that this glitch only exists in the Japanese versions of the game, or is otherwise a glitch from a Pokémon game which was only released in Japan.
Training a Pokémon without having to send it into battle
The partial switch glitch is a Select glitch which only occurs in v1.0 of Pokémon Red and Green, due to the developers patching out the ability to use a Select glitch solely from within battle in later versions (however other Select glitches are still possible if brought over from outside of battle).
When you press Select on an item outside of battle and go into battle, it keeps the cursor position exactly for the other menus (moves and PkMn).
For example, if you have six Pokémon and then press select outside of battle on the fifth item, exit the menu with B,B then enter battle when you choose 'PkMn' the cursor is highlighted on your fifth Pokémon and you can 'swap' it. If you don't have a fifth Pokémon, you'll swap a glitch Pokémon and trigger another select button glitch, but if you swap around a legitimate Pokémon with the first Pokémon in the party (or currently in battle), you can train one of them without battling with it and without the need of an Exp. All.
When you swap different party Pokémon when already in battle, you'll still use the former Pokémon and you're free to defeat the enemy Pokémon with it, although the other Pokémon will gain the experience and receive the current HP of the former Pokémon plus any stat changes if it levels up in the process.
This glitch can also be used to quickly faint stronger Pokémon, e.g. if you partially switched a Level 100 Mewtwo into Pidgey's when Pidgey (Level 5) was already in battle, if Pidgey faints it can actually be used again, but Mewtwo will faint in its place.
- ChickasaurusGL (video/article description)