After you Multi-Shot, your pet's melee attacks also strike all other nearby enemy targets for 50% as much for the next 4 sec.
Beast Cleave is a passive hunter ability learned at level 24 for hunters who have chosen the Beast Mastery specialization. It allows the hunter's pet to AoE strike after the hunter uses [Multi-Shot].
Patch changes
Patch 5.2.0 (5-Mar-2013): Beast Cleave now causes the pet to strike all other nearby targets for 50% damage (was 30%).