Counterpunch

Combat

With deft reflexes, you capitalize on opponents’ mistakes.

Prerequisite: Dex 18, Combat Reflexes, Improved Unarmed Strike, Weapon Focus (unarmed strike); base attack bonus +16 or brawler† level 12th.

Benefit: Once per round, when you are fighting unarmed with both hands free and an opponent misses you with a melee attack, the opponent provokes an attack of opportunity from you. You must use an unarmed strike for this attack of opportunity.

Source: Advanced Class Guide