On September 16, 1994, at 1530 central daylight time, a Rotorway Exec 90 homebuilt helicopter, N92RE, was substantially damaged while landing at Gundy's Airport, Owasso, Oklahoma. The helicopter, flown by a helicopter instructor pilot, was on an instructional flight. There was no flight plan filed and visual meteorological conditions prevailed. The pilot and student were uninjured. The instructor pilot reported the following information. The student was practicing hovering using the collective, throttle, and anti-torque pedals while the instructor controlled the cyclic. A left yaw developed and the instructor noticed the engine RPM in the lower portion of the green arc. The instructor then added full throttle and right pedal, but the left yaw continued to increase in severity. At this point the collective was lowered until the skids contacted the ground. The right skid contacted first, the helicopter then rolled to the right, severing one main rotor blade and bending the tailboom. The aircraft came to rest on its right side. The instructor pilot also stated that the hover seemed normal until the left yaw occurred and there were no low RPM indications such as collective position or sound. He believed the yaw was caused by either low main rotor RPM or possibly by low tail rotor RPM resulting from belt slippage or an unknown cause. An examination of the helicopter by a Federal Aviation Administration inspector revealed no mechanical anomalies that could have contributed to the mishap.