Deadeye Shot

 
Prerequisite(s): Base Attack Bonus 4+, Skirmish or Sneak Attack Class Ability, Point Blank Shot, Precise Shot
 
Benefit: By firing just as your ally connects with a blow, you take advantage of the distraction to strike the same enemy when his guard is ruined. If you ready a ranged attack to occur when an ally strikes a particular enemy, and your ally succeeds, that target loses his Dexterity bonus to his AC against your attack.
 
Source: Players Handbook II