SRD:Prowess

If an enemy provokes an attack of opportunity from you, you can make the attack even if you’ve already taken your allotted number of attacks of opportunity this round (usually one).

You can manifest this power instantly, quickly enough to gain an extra attack of opportunity in the same round. Manifesting this power is an immediate action, like manifesting a quickened power, and it counts toward the normal limit of one quickened power per round.