Giovanni and Red only work if the attack does damage

mrtwrecks

I don’t really understand this mechanic. I had Shuckle ex in the active spot and used Giovanni. I flipped three tails on the attack, so unfortunately didn’t do any damage. However, Giovanni is supposed to do 10 damage to the opponent. Why does only apply the damage if the attack causes damage? The damage is supposed to be in addition to the damage done from the attack. 0 + 10 = 10. It’s certainly not a multiplier which would make sense considering 0 x 10 = 0. But it’s not a multiplier, considering when you do 30 damage, the card does not do an additional 300 damage. Is there an explanation/reason for this?