During a routine training flight, the helicopter was in the traffic pattern practicing power recovery auto-rotations, with the pilot under training at the flight controls. During the 5th practice auto-rotation, the flight instructor reduced the throttle, and the helicopter began a normal descent. During the deceleration flare, about 75 ft above ground level, the flight instructor heard the low rotor RPM audio tone. Using a positive transfer of control, the flight instructor quickly took the flight controls. He increased the throttle to what he believed to be the “FLY” position. The helicopter settled onto a taxiway. The main rotor blades severed the vertical fins on the horizontal stabilizer, and the tail rotor driveshaft was severed. The operator reported that the pilot did not rapidly roll the throttle to the “FLY” position during the deceleration/flare, and the helicopter contacted the ground at less than 100% rotor RPM. The operator stated that there were no mechanical anomalies with the helicopter that would have precluded normal operations.