A Rotation Battle consists of three Pokémon outside of their Poké Balls, similar to a Triple Battle, but only one Pokémon can attack at a time. Unlike switching or moving positions in a Triple Battle, rotating Pokémon is a free action, meaning both players may rotate and attack in the same turn.
In the games
When a Rotation Battle begins, the first Pokémon able to battle in the player's party will be sent out to the primary position. The second member of the party will be counterclockwise from the first, while the third will be clockwise from the first.
Only the active Pokémon plays any part in the battle. Moves and Abilities that affect more than one Pokémon will only affect the opponent's active Pokémon, and Abilities like Flower Gift will only activate if the user is active. Status damage is not received while a Pokémon is inactive.