From the Super Mario Wiki
Pokémon Change is the Pokémon Trainer's down special move, and by extension the down special move of Charizard, Squirtle, and Ivysaur, in Super Smash Bros. Brawl. The move is based on the ability for players to switch out their Pokémon mid-battle in the Pokémon series, allowing players to switch between using Squirtle, Ivysaur, and Charizard in that order. Pokémon Change is also automatically used whenever one of the Pokémon's stamina is depleted or whenever one is KO'd. This move cannot be used immediately after respawning, or at all in midair. Status effects are also not carried over between Pokémon, though damage percentages are, and a Pokémon's stamina recharges while it is not in use.