text: If an opponent casts a spell with X in its mana cost, the mana value of that spell takes the value of X into account. If you reveal a card with X in its mana cost, X is 0. For example, if your opponent casts Blaze with X=1, Counterbalance will counter that spell if you reveal a card with mana value 2, but it won't counter that spell if you reveal a Blaze of your own.