Impetuous/Memories That Stain Its Armor both respond "after your opponent play an ability on a foundation".
Question is, then: what happens in the following situation?
1. Player A uses an ability commit something in Player B's staging area.
2. Player B responds with Torn Hero, pays cost.
3. Player A responds with Impetuous/Memories to destroy Torn Hero.
Does Torn Hero's static ability go into effect, even though it was never committed? I assume it still takes the place of the card that would have been committed, of course - just wasn't sure about the other.