The handle engages the chain brake of the STIHL chainsaw: when pushed by the hand or on kickback, it tightens the brake band and stops the chain.
Replace it if it is cracked, broken, the mount has loosened, or the brake does not engage crisply. Checking the brake before work should be a habit.
It is best to do the work on a cooled engine with the ignition off, and to tighten the fasteners without overtightening, so as not to strip the thread in the body.
