Kickback
The violent upward and backward rotation of the guide bar towards the operator, caused when the upper quadrant of the bar nose — the kickback zone — contacts wood, or when the chain is pinched in the top of the cut. It is the leading cause of serious chainsaw injuries.
Kickback happens faster than human reaction time, so it is prevented by technique and equipment rather than reflexes: keep the nose of the bar aware and out of unseen contact, cut with the underside of the bar wherever possible, stand slightly to the side of the cutting plane, and grip the front handle with the thumb wrapped underneath. Low-kickback chains with bumper drive links or guard links, reduced-radius bar noses and a working chain brake all cut the severity. A blunt chain makes kickback more likely because it encourages you to force the saw into the wood.