Stalwart Defense( Player's Handbook II, p. 83)
You excel at aiding your allies in battle. When an opponent attempts to strike one of them, you make a quick, distracting motion to ruin the foe's efforts.
When an opponent you threaten attempts a melee attack against one of your allies, you can give up one of your attacks of opportunity to attempt an aid another action. You target the attacking opponent with the aid another. The target of your foe's attack gains the benefit of your action in the form of a +2 bonus to AC against the attempted attack. You cannot use this feat against the same opponent more than once per round.