His girlfriend is about to explode, and he's at ground zero, often without a bomb kit or a prayer. Maybe he honestly did something wrong, like cheating with another girl. Maybe the compromising position she found him in isn't really what it looks like (anything from being assaulted by a Clingy Jealous Girl to suffering an Accidental Pervert moment or even a completely Unprovoked Pervert Payback). No matter what the cause, she's hurt by his (apparent) betrayal and she won't stop until he feels her pain. And so, the slap effectively pierces his armor. Examples of Armor-Piercing Slap include: