Seeing we are discussing the S76--going back to the very beginning with the Allisons.
To brighten up a boring trip the Allisons had an overspeed protection device. You tested this when taxiing out by pulling one engine back until the needles split and then holding down its test switch. This would cause the engine to hunt about 10% up and down proving the system worked. This used to go off in the air so you had one engine galloping up and down with the other galloping down and up trying to compensate. They would never get in sequence so the aircraft would fishtail furiously with the swings in torque. One had to lower the lever and pull the throttles to see which one had gone wrong before it threw the passengers out of the windows. Pulling it back to idle cleared it and so on you went.