On the B206, if the engine to txmsn driveshaft fails, you still have the drive going back to the tail rotor, which will initially go berserk with the overspeed. The N1 governor should stop the N1 at 105%. Some other aircraft have an overspeed trip and will kindly shut the engine down. Not really needed, if the overspeed is caused by a faulty sensor and it isn't overspeeding at all.
Do not shut down the 206 engine for your auto, or you will lose all tail rotor control, get to the bottom with a big spin, sideways impact, stall flick spin crash burn die.