The pilot, who was also a certified helicopter flight instructor, was demonstrating a 180-degree autorotation to the flight instructor as part of a flight review. While on final glide approximately 100 feet above ground level (agl), the landing checks were complete to include: rotor RPM in the green arc, airspeed at 60 knots, and the landing skids aligned with runway. Approximately 60 feet agl, the rotor RPM was at the bottom of the green arc, and the flight instructor expected the pilot to initiate a flare; however, the flare was "very mild, if none, with little reduction in aircraft speed and rate of descent." Subsequently, the flight instructor "hesitated" in taking the helicopter controls and the helicopter landed hard and skidded down the runway. During the hard landing, the main rotor blades severed the tail boom. Examination of the airframe revealed no anomalies that would have precluded normal operations.