The first shot you fire in a round, you notch two arrows instead of one. Both arrows use the same attack roll, but the attack roll is made at a -4 penalty. If the attack roll hits, the target takes damage from both arrows (extra damage, as from sneak attack and/or critical hits, are only applied to one of the arrows). When your base attack bonus reaches +11, the first and second attacks you make in the round fire two arrows, and the attack roll penalty increases to -6. When your base attack bonus reaches +16, the first three attacks you make in the round fire two arrows, and the attack roll penalty increases to -8.