It requires BAB +4, PBS, Precise Shot, and sneak attack or skirmish. Feat lets you choose an enemy to ready a ranged attack against if it is struck by an ally in melee. When/if that occurs, you catch the foe flatfooted. So, a decent way for a ranged rogue to guarantee one SA/round. Of course, all that readying means lots of delayed turns, so over the course of several uses you may also effectively lose a turn