If your opponent's Bench isn't full, look at his or her hand. If your opponent has any Baby Pokémon or Basic Pokémon there, choose 1 of them and put it on his or her Bench. Then, switch it with the Defending Pokémon.
If there are any Darkness Energy cards attached to Dark Houndoom, discard 1 of them and this attack does 30 damage plus 20 more damage (plus 10 more damage for the Darkness Energy you discarded). If there aren't any, this attack does 30 damage.