As per the Dungeon World SRD:
n Piercing: It goes right through armor. When you deal damage with n piercing, you subtract n from the enemy's armor for that attack.
So the idea is that a piercing attack will ignore armor to some extent. But what happens when the piercing exceeds the armor rating of whoever is taking the blow?
As far as I recall, there isn't anything that says whether or not armor can drop below zero. If I were to apply the piercing tag's "subtract n from ... armor" strictly, then that would essentially mean additional damage.
Of course, the other argument is that the leftover (n - armor) piercing does not add damage because the attack's normal damage is simply dealt as if unhindered (assuming that either armor cannot be negative, or that negative armor is not factored into damage calculations).
So the crux of my problem is: Is there a rule that specifies if Armor cannot be negative?