On May 27, 2007, about 1045 Pacific daylight time, a McDonnell Douglas Helicopter Company 500-E, N526BH, impacted flat desert terrain while practicing autorotations at the Flying-M-Ranch, Yerington, Nevada. Air Finance Corporation operated the helicopter as a training flight under the provisions of 14 CFR Part 91. The helicopter sustained substantial structural damage. The certified flight instructor (CFI), and student pilot sustained minor injuries. Visual meteorological conditions prevailed for the local area instructional flight, and no flight plan had been filed. According to the CFI, they had been working on emergency procedures (engine failures) at low altitudes; procedures started at 100 feet agl with successful recoveries at 20 feet agl. They had completed four successful 180-degree power recovery autorotations, as well as three simulated low altitude engine failures with power recovery. The CFI believed that the student had responded properly to the emergency procedures. As they setup for another low altitude failure, the CFI noted that the flight departed the height/velocity (HV) curve to an approximate airspeed of 60-70 knots. He instructed the student to simulate an engine failure by rolling off the throttle to flight idle (as they had done previously). The CFI stated that after the student rolled off the throttle, the student lowered the collective. As the helicopter began to descend, the CFI visually checked the rotor/engine tachometer instrument, and simultaneously heard the engine spooling down. The CFI reported that there was no split between the rotor and engine tachometer needles. He suspected that the collective had not "bottomed out completely" and immediately took over the flight controls (collective, cyclic, and pedals). He rolled the throttle to full flight rpm and lowered the collective. He also raised the nose of the helicopter hoping to build inertia in the blades, trying to regain some of the decayed rotor rpm's. The CFI realized that the engine would not have adequate time to spool up. He rolled the throttle off to flight idle and cushioned the landing by leveling the skids and controlling the yaw moment with the pedals, while simultaneously raising the collective until it was in the full up position. The helicopter landed hard with forward speed before coming to rest upright on the desert floor. According to a Federal Aviation Administration (FAA) inspector, who examined the helicopter on scene, there were no mechanical problems noted with the helicopter. The main rotor blades remained attached at the main rotor mast; however, the blades displayed a downward deformation. The tail boom had separated.