Flip a coin. If heads, this attack does 20 damage. If tails and if your opponent has any Benched Pokémon, he or she chooses 1 of them and this attack does 20 damage to it. (Don't apply Weakness and Resistance for Benched Pokémon.)
Disclaimer: The following data is based on sale history from a third party source. Data is updated about every 5 days. This data does not serve as a value guarantee.