As long as Beautifly's remaining HP is 40 or less, Beautifly does 40 more damage to the Defending Pokémon (before applying Weakness and Resistance).
Before doing damage, you may choose 1 of your opponent's Benched Pokémon and switch it with 1 of the Defending Pokémon. If you do, this attack does 20 damage to the new Defending Pokémon. Your opponent chooses the Defending Pokémon to switch.
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