Muzurhythm the String Djinn (YS12-EN040) [Starter Deck: Xyz Symphony]

2 Level 3 monsters // During the Damage Step, when a "Djinn" Xyz Monster you control is attacking an opponent's monster: You can detach 1 Xyz Material from this card; the attacking monster's ATK is doubled until the End Phase. The effect of "Muzurhythm the String Djinn" can only be used once per turn.