Hindering Opportunist
( Player's Handbook II, p. 79)[General]
When you have a chance to strike a distracted foe, you instead use that opportunity to aid or protect an ally against him.
Prerequisite
Combat Reflexes (PH) , base attack bonus +3,
Required for
Stalwart Defense (PH2) ,
Benefit
Whenever an opponent provokes an attack of opportunity from you, you can choose to replace that attack with an aid another action targeting that opponent