The helicopter flight instructor and private pilot were conducting an approach to a pinnacle landing pad. Two previous passes were made and both pilots estimated winds to be from the northeast at 8 knots. While approaching the landing pad from the southwest, the flight instructor noticed that the helicopter's rpms were slightly low and he reminded the private pilot to increase power until the rpms were in the green arc of the rpm gauge. The private pilot did not increase power sufficiently enough to place the rotor rpm in the green arc and the helicopter began to descend rapidly. The flight instructor took control of the helicopter, increased power, and attempted to reverse course and travel downhill. The helicopter landed hard, bounced, and rolled approximately 1 1/4 times before coming to rest on its right side and sustaining substantial damage to its fuselage and tail boom. An examination of the airframe by a Federal Aviation Administration inspector did not reveal any preimpact anomalies. Additionally, neither pilot reported a malfunction of the airframe or engine.