Reinterpret

If a spell with X generic mana in its mana cost is countered this way, X generic mana is the value chosen as the spell was cast.

If you cast a spell “without paying its mana cost,” you can’t pay any alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.

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.