Box trick

The Box trick is a method in the 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 it's own base stats. But by depositing a Pokémon into your PC and withdrawing it, the game will, in the process of depositing\withdrawing, recalculate the stats of your Pokémon based on the amount of effort points etc. it possesses.