On May 7, 1993, at 1715 hours Pacific daylight time, a Robinson R-22, N4071G, collided with trees in mountainous terrain about 5 miles southwest of Corona, California. The pilot was conducting a local visual flight rules personal flight. The helicopter, operated by Parflite, Inc., Riverside, California, was substantially damaged. The certificated commercial pilot was not injured; the passenger received minor injuries. Visual meteorological conditions prevailed. The flight originated at the Riverside Airport at 1700 hours. The pilot reported the helicopter sustained a loss of engine and main rotor RPM while in forward flight at about 20 to 30 knots between 100 to 150 feet above trees in rugged mountainous terrain. The pilot indicated that the main rotor RPM dropped about 3 to 4 percent to 101 percent. The pilot increased the throttle in an attempt to regain the rotor rpm, but without success, and the helicopter began to descend. The pilot noted that the engine manifold pressure was indicating 27 inches during the loss of rotor RPM. The pilot turned the helicopter away from uprising terrain, and reduced the collective and increased the throttle in order to regain the main rotor RPM. The engine and rotor RPM did not increase and the low rotor RPM warning horn and caution light activated. The pilot selected a forced landing area in the forested rugged mountainous terrain. The helicopter descended into the trees, struck the ground and slid down slope on its left side. The Robinson R-22 is equipped with a low rotor RPM warning horn and caution light. The low rotor RPM warning horn and caution light indicate rotor RPM at 95 percent or below. The horn stops and amber caution light extinguishes when the RPM is increased to safe limits or when the collective control is full down. Robinson Helicopter Company publishes information on the R-22 helicopter that states that at airspeeds below 53 knots the helicopter is considered on the "back side of the power curve" and that more power is required to be provided to the rotor system. The helicopter emergency procedure for activation of the low rotor RPM warning horn and caution light states "Immediately lower collective." The helicopter was examined at National Aircraft Transportation, Inc. facilities in Long Beach, California, after the accident. There was no evidence of any mechanical failure or malfunction found.