Once per turn, during your Main Phase, you can equip it to "X-Head Cannon" you control as an Equip Card, OR unequip it to Special Summon this card in face-up Attack Position. While equipped to a monster by this effect, that monster gains 400 ATK and DEF. (A monster can only be equipped with 1 Union Monster at a time. If the equipped monster would be destroyed by battle, destroy this card instead.)