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).

Luring Antenna

20

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.