You carefully line up a ranged attack, timing it precisely so that you hit your opponent when his guard is down. When your target is busy dealing with an ally's melee attack, you strike.
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 target, and your ally succeeds, that target loses his Dexterity bonus to AC against your attack.
