Your expert use of your shield allows you to strike at vulnerable foes even when you forgo your own attacks in favor of defense.
Prerequisites: Shield Proficiency, Shield Specialization.
Benefit: When fighting defensively and using a shield, you do not take the standard fighting defensively penalties on attacks of opportunity.
When using the total defense action and a shield, you still threaten the area around you as normal. You can make attacks of opportunity with a –4 penalty.
Normal: You take a –4 penalty on all attacks while fighting defensively. You cannot attack while using the total defense action.
Special: A fighter can select Active Shield Defense as one of his fighter bonus feats.
Source: Player’s Handbook II, page 71.