During damage calculation in your opponent's turn, if this card is being attacked: You can target the attacking monster; make that target's ATK 0 during damage calculation only (this is a Quick Effect). This effect can only be used once while this card is face-up on the field.
After reading this evaluation I stopped believing in this websites proficiency about other cards. "Blue-Eyes White Dragon is better than this card in terms of stats, and you will not use this card." Really? Seriously?
I've got a deck completely based around this card: unshaven angler to and the spell that summons 2 tokens, then set suijin, use spells to boost his def and wait for the enemy to attack, then you can reduce enemy atk to 0 and deal up to 4700 damage