Hindering Opportunist

(Player's Handbook II, page 79, DnD 3.5)

[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, base attack bonus +3

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