On June 24, 2001, approximately 1930 Pacific daylight time, a Hughes 269A single engine helicopter, N8724F, was substantially damaged during a hard landing following an autorotational landing at the Paso Robles Municipal Airport, Paso Robles, California. The flight instructor sustained serious injuries and the student was not injured. The helicopter was registered to and operated by Del Rio Aviation, Paso Robles, under the provision of 14 CRR Part 91 as an instructional flight. Visual meteorological conditions prevailed for the flight, and a flight plan had not been filed. The local instructional flight originated about 1845. According to the instructor pilot, he was demonstrating a simulated loss of engine power and ensuing autorotational landing to the student. He reduced the throttle and lowered the collective. He then pointed out an area to land to the student. As he looked back into the cockpit (approximately 400 feet agl), he noticed the rotor rpm had decreased "drastically" and was dropping even lower. The instructor then ensured the collective was lowered all the way and put the cyclic forward; however, he was unable to regain the lost rotor rpm in time. The instructor then flared the helicopter, but it impacted the ground hard. The main rotor blades severed the tail boom and the helicopter rolled over. The flight instructor indicated he was not sure how the rotor rpm got so low, but commented there were no problems with the engine or the freewheeling unit. The instructor pilot reported having accumulated approximately 2,000 total hours in helicopters, of which about 300 hours were flown in the accident helicopter make and model.