Your critical Aimed, Steady and Chimera Shots cause the target to bleed for 30% of the damage dealt over 8 sec.
Piercing Shots is a passive hunter ability learned at level 72 for hunters who have chosen the Marksmanship specialization. Its bleed effect is triggered by [Aimed Shot], [Steady Shot], and [Chimera Shot] criticals.
Patch changes
Hotfix (2009-04-17): "Piercing Shots will now cause the correct amount of damage against shielded targets. [1]"
Patch 3.1.0 (14-Apr-2009): Re-designed: Your critical Aimed, Steady and Chimera Shots cause the target to bleed for 10/20/30% of the damage dealt over 8 sec.