On September 23, 2002, about 1000 Pacific daylight time, a Robinson R22B, N2331X, impacted hilly terrain during an off-airport forced landing about 4 miles east of Silverado, California. Neither the commercial pilot, who possessed a certificated flight instructor (CFI) certificate, nor the private pilot was injured during the instructional flight. The helicopter was substantially damaged. The helicopter was operated by Helistream, Inc., Santa Ana, California, under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed at the time, and no flight plan had been filed. The flight originated from Santa Ana about 0945. The operator's manager reported to the National Transportation Safety Board investigator that the CFI's student was, in part, practicing landings. During approach in a confined mountainous area, low main rotor rpm was experienced and the CFI took over the controls. The terrain elevation was about 2,900 feet mean sea level. Despite the CFI's efforts, he was not able to regain rotor rpm. The helicopter touched down on slopping terrain short of the intended helipad. During the touchdown sequence, the main rotor blades contacted the hillside, and the helicopter's fuselage yawed about 360 degrees before coming to rest in an upright attitude with a bent tail boom. In the CFI's completed accident report, he indicated that during the approach he heard the low rotor rpm horn. Immediately thereafter, he looked at the rotor rpm gauge and lowered the collective while rolling on the throttle. The CFI stated that the rpm did not increase. Then, he added a "large amount of throttle while (again) lowering the collective, but the rpm continued to steadily decrease." The CFI also indicated that in the vicinity of the accident site the outside air temperature was about 95 degrees Fahrenheit. The wind was calm. No mechanical malfunctions or failures were reported with the helicopter. Federal Aviation Administration airman certificate and medical documents dated July and August 2002, indicate that the CFI and the private pilot's combined weights were approximately 358 pounds. The CFI reported that 23 gallons of fuel were on board upon takeoff for the accident flight. The operator stated that the helicopter was flying near its maximum certificated gross weight. Based upon the atmospheric conditions provided by the CFI/operator, the density altitude was calculated to be in excess of 5,800 feet.