Your successful Chimera Shot and Multi-Shot attacks daze the target for 4 sec.
Concussive Barrage is a passive hunter ability learned at level 30 for hunters that have chosen the Marksmanship specialization. It causes [Chimera Shot] and [Multi-Shot] to daze the target for 4 seconds.
Patch changes
/Patch 3.0.2 (14-Oct-2008): Can now proc from Volley and Multi-shot attacks.
Patch 3.3.0 (08-Dec-2009): This ability is no longer subject to spell reflects.
Patch 3.1.0 (14-Apr-2009): Concussive Barrage redesigned: 2-point talent. Now causes your Multi-Shot and Chimera Shot to have a 50/100% chance to daze the target for 4 seconds.