Attack of Opportunity

Trigger A creature within your reach uses a manipulate action or a move action, makes a ranged attack, or leaves a square during a move action it's using.

You lash out at a foe that leaves an opening. Make a melee Strike against the triggering creature. If your attack is a critical hit and the trigger was a manipulate action, you disrupt that action. This Strike doesn't count toward your multiple attack penalty, and your multiple attack penalty doesn't apply to this Strike.