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?
"BEWD is straight up better than this card"
Uh, no it's not. Even with support, Blue-Eyes dies in a straight battle to stronger monsters. Suijin can, once while it's face-up, destroy anything in combat without spells or traps.
Check out how to build a deck with this card!
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