On September 9, 1999, about 1130 hours Pacific daylight time, a Robinson R-22-Beta helicopter, N4076Z, sustained substantial damage during a hard landing while practicing autorotations at Piute, Nevada. Extreme Machines Helicopter Services operated the helicopter under the provisions of 14 CFR Part 91. The two certified flight instructors who rented the helicopter sustained minor injuries. The personal flight departed North Las Vegas about 1030. Visual meteorological conditions prevailed and no flight plan was filed. The accident site was located at 36 degrees 20 minutes north latitude and 115 degrees 17 minutes longitude. The instructors stated they were reviewing advanced emergency procedures. They flew to a practice-landing pad and completed a normal landing to determine winds and survey the landing area. They practiced six straight-in autorotations and completed five successful 180-degree autorotations, alternating pilot-in-command duties each time. The last planned autorotation began at 600 feet above ground level (agl) and 70 knots. The pilot said that turning into the landing area he was maintaining rotor rpm (revolutions per minute) in the green arc and the airspeed at 60 knots. As he was about to roll onto final, he heard the low rotor rpm horn and saw the rpm drop to 85 percent. As he lowered the collective, he noted he was still descending rapidly and he did not see the rpm increase substantially. He attempted a power recovery and the other pilot tried to help roll on the throttle and lower the collective. At 100 feet with full throttle and the collective lowered, the pilot recognized he would touch down and attempted to flare and lower the descent rate. He stated that the rotor rpm remained below 80 percent and the airspeed slowed to 40 knots. He raised the collective to cushion the landing and touched down an estimated 200 feet from his intended landing spot. The helicopter landed hard and began to slide. The right skid collapsed, the helicopter rolled onto its right side, and twisted 180 degrees. After sliding approximately 75 feet, the helicopter came to rest heading the opposite direction. Neither the pilot nor the operator submitted a pilot/operator report, NTSB Form 6120.1/2. Certified mail sent to the addresses provided was returned marked undeliverable.