What to do when you "return a Pokemon to your hand"
The mechanics for this kind of attack on Pokemon Trading Card Game Online (PTCGO, Official Pokemon TCG) is that whenever a Pokemon uses a 'Return this Pokemon...' attack, the damage is inflicted to the defending Pokemon and then the attacking Pokemon (along with attached cards, this also means, if it's a Stage 2 Pokemon, also take the stage 1 and Basic Pokemon) goes back to the owner's hand. The owner must then choose a benched Pokemon to be their new active Pokemon. In the case that there is no benched Pokemon, you cannot use this attack as this will leave you with no active Pokemon and will cause you to lose the game.