On an earlier post, we discussed the action/reaction effect of the main rotor on the fuselage, necessitating the use of the tail rotor on most helicopters. (Tandem rotor helicopters like the CH-47 Chinook and the CH-46 Sea Knight have two rotors that turn in opposite directions, thereby cancelling out the torque effect.)
Should the tail rotor become ineffective for any reason, the result is an uncontrolled turn of the fuselage in the direction opposite the main rotor .
The following video graphically shows loss of tail rotor effectiveness. It's difficult, watching this video, to tell if this loss is due to stalling of the tail rotor, or a malfunction of some component in the tail rotor drive system. It doesn't really matter, because the malfunction shown follows "Murphy's Law" by happening at EXACTLY the wrong instant, when the aircraft is most vulnerable!