Reinterpret

If you target a spell that can’t be countered, like Heated Debate, you won’t counter the spell, but you will get to cast a spell with an equal or lesser mana value.

If the spell you cast has {X} in its mana cost, you must choose 0 as the value of X.

This can be used to cast either face of a modal double-faced card, but only if the face you are casting has an equal or lesser mana value than the target spell.