Your guard has a hidden health bar.
Your guard has a hidden health bar that equals 100 points. Each normal attack does 5 points, meaning you have to land 20 normal attacks (including most specials) to break a shield. Fully charged heavy attacks break it instantly, and dashes do about 25 damage to shields. Special startups do fifty. Although you'd be losing a health bar, using a meter to break someone's shield could turn the tide.