The helicopter was destroyed by impact forces and a post crash fire while attempting an auto-rotation after a mechanical failure. The pilot stated that he was practicing takeoffs, hovering, and quick stops above the runway. After takeoff, at about 50 feet and 50 mph, he lowered the collective to initiate a quick stop. At this point the engine RPM revved up out of control. He pulled up on the collective to re-engage the drive system, but the system would not engage. He entered into an auto-rotation, and as he neared the ground the helicopter began to slide sideways, folding the skids under the helicopter. The helicopter then rolled on its side, and the occupants climbed out prior to the post crash fire. Subsequent examination of the helicopter's drive system revealed that the drive shafts, pulleys, and drive belts were intact. The only device within the helicopter drive system that could not be determined to be in working condition was the sprag clutch which transmits engine power to the rotor drive system. The sprag clutch was not examined during the course of the investigation.