Forretress MODN2 21

During your opponent's turn, whenever 1 of your opponent's Benched Pokémon becomes his or her Active Pokémon, Forretress does 10 damage to it. (Don't apply Weakness and Resistance.) This power stops working if Forretress is affected by a Special Condition.

If your opponent has any Benched Pokémon, he or she chooses 1 of them and switches it with his or her Active Pokémon, then, if you have any Benched Pokémon, you switch 1 of them with your Active Pokémon. (Do the damage before switching the Pokémon.)