According to the helicopter flight instructor, he was preparing a helicopter-rated commercial pilot for his helicopter flight instructor check ride. After the pilot receiving instruction completed a straight-in autorotation, the flight instructor demonstrated a throttle-off, 180-degree autorotation from 600 feet above ground level (agl). The pilot receiving instruction then attempted the same maneuver, but during the entry, the helicopter airspeed became slow while the rotor rpm remained "in the green." The pilot receiving instruction lowered the nose of the helicopter, which increased airspeed. As the helicopter turned onto final approach at an altitude of about 300 feet agl, the flight instructor advised the pilot receiving instruction to ease back on the cyclic while raising the collective to maintain rotor rpm. About 150 feet agl, at an airspeed of about 85 knots, and a rate of descent of about 2,000 feet per minute, the flight instructor took over control of the helicopter and began to pull back on the cyclic while raising the collective. Despite the flight instructor’s efforts, the helicopter continued a tail-low descent into the ground, severing the tail boom. Postaccident examination revealed no preexisting mechanical malfunctions or anomalies that would have precluded normal operation.