On January 12, 1994, about 1415 Atlantic standard time, a Robinson R22, N8044Y, registered to Hill Construction Corporation, doing business as Hill Aviation, operating as a 14 CFR Part 91 instructional flight, crashed while terminating an autorotation with turn and a power recovery. The helicopter sustained substantial damage. The certified flight instructor and student pilot were not injured. The flight originated about 45 minutes before the accident. The certified flight instructor (CFI) stated the student pilot initiated a 180 degree autorotation with turn at about 500 feet agl. The intended touchdown point was a dirt road. The student pilot overshot the turn while at about 90 feet agl. The CFI allowed the maneuver to continue to a grassy area located to the right of the dirt road. A flare was initiated at about 40 feet agl. A power recovery (increase in throttle), and initial collective pitch application was made at about 4 to 6 feet agl above 3 foot tall grass. The throttle did not respond. The CFI got on the flight controls with the student pilot. The helicopter collided the ground with about 12 to 15 knots forward airspeed, and rolled over on it's right side. An engine examination and engine run was conducted by the FAA on January 13, 1994. There was no evidence of a mechanical failure or malfunction. The Robinson R22 Flight Training Guide states on page 23: NOTE As the aircraft descends through 100 feet AGL, make an immediate power recovery if the following conditions do not exist: 1. Aircraft aligned with the touchdown point 2. Rotor RPM in the green 3. Airspeed 60 to 70 KTS 4. Rate of descent less than 1500 FPM