The student pilot, who held a commercial fixed-wing pilot certificate, was performing hovering flight during his first helicopter lesson when the helicopter "suddenly jerked backwards." The helicopter's tail rotor gearbox and a portion of the tail rotor departed the helicopter, and the flight instructor landed the helicopter upright with no injuries to the two occupants.According to the flight instructor, he monitored the cyclic, collective, and tail rotor controls while explaining hovering flight, and surrendered each control to the student pilot one at a time until the student pilot was fully controlling the helicopter. As the helicopter transitioned backwards, and began a "spin to the left," he requested and took control of the helicopter, but the student pilot did not surrender the controls, and the flight instructor could not overpower the student pilot's inputs before the tail rotor collided with terrain. The flight instructor further stated that there were no mechanical deficiencies with the helicopter that would have precluded normal operation.