So you get +3 power and use attack. Your chosen foe draws one card les when getting a new hand, 5 cards are normal so 4. If he still draws the 5th card and you tell him that he forgott before his turn ends, you defeat him. Which means you kill him regardless of how many hp he has.
Or you can use this as a defense.
You dont get the power, you discard the card and draw a new one. Also the attacker now is the target of his attack.