DIAGNOSIS The motor is getting power and trying to turn, but it’s mechanically bound up. That humming is the sound of a stalled brushless motor. The chain brake isn’t the issue here—this is usually a physical jam in the drive system.
FIX Immediately release the trigger. First, check for the obvious: is the chain so tight it’s frozen on the bar? Loosen the bar nuts and back off the tensioning screw. Can you move the chain freely by hand now? If not, the jam is likely in the drive sprocket or clutch area. Remove the bar and chain completely. Now try pulling the trigger (safely, pointing away). If the clutch drum spins freely, the problem was your bar/chain setup. If it still hums and jitters, the jam is internal.
IF THAT DIDN'T WORK With the bar and chain off, inspect the clutch drum and drive sprocket for debris like a wood chip, a broken drive link, or a badly worn sprocket that’s binding. Also, manually try to turn the clutch drum with your fingers (with the battery out). If it won’t turn, the clutch assembly or motor bearings have seized, which is a serious repair.
HEADS UP Don’t hold the trigger down when it’s making that humming sound. You’re forcing the electronics to push maximum current into a stalled motor, which generates a lot of heat and can permanently damage the motor’s windings or the field-effect transistors (FETs) on the circuit board. Short, diagnostic trigger pulls only.