Box trick
The Box trick is a method in the Generation I and II Pokémon games that allows a Pokémon to gain effort points without necessarily leveling up in the process. This method is most useful when training a Pokémon that is already at level 100.
Usually, a Pokémon gains stat points when they level up, based on the amount of effort points that the Pokémon has acquired through battle, and based on its own base stats. But by depositing a Pokémon into the PC and withdrawing it, the game will, in the process of depositing/withdrawing, recalculate the stats of the deposited Pokémon based on the amount of effort points etc. it possesses. The reason it does this is because while in the box the actual stat values are not stored, so they must be calculated when taken out or viewed.
A Level 100 Pokémon in Generation III and IV is unable to gain effort points, so this trick cannot be used to increase a Level 100 Pokémon's stats in those games. However, a Pokémon can still gain effort points through this method in later generations, provided that they are not level 100.
