When a Pokémon is introduced to the game, it will have a Stealth Rock hazard placed on its side of the field. When a Pokémon from the opponent's side switches in, that Pokémon will take damage based on how weak or resistant it is to Rock-type attacks. This damage range starts at 12.5% and increases by 1.25% for every consecutive turn the Pokémon remained in the game (multiplicative). For example, if a Pokémon was introduced to the game and had a Stealth Rock hazard active on its side of the field for 3 turns, it would take 37.5% damage each time it switched in.