When your opponent Special Summons a monster(s) (except during the Damage Step): You can Special Summon this card from your hand in face-down Defense Position, and if you do, during the End Phase of this turn, change it to face-up Defense Position. You can only use this effect of "Sneaky "C"" once per turn. If this card is flipped face-up during the End Phase: Destroy all Special Summoned monsters on the field.