Genuine Daikin 139863J Outdoor Fan Blade is the propeller blade that pulls air across the outdoor coil so the condenser can release heat. When a blade is damaged or thrown out of balance, the outdoor unit tends to vibrate, hum or knock, and shift less air than before. The genuine blade carries Daikin's own profile and balance, so it spins true and quiet rather than putting strain on the motor bearing the way an ill-fitting blade can.