The pilot intended to land on a small knoll in the centre of a large cleared area in heavily timbered country. During the flare to land over a large depression, the pilot noticed a lack of power accompanied by the Low Rotor RPM Warning Horn. He lowered the collective control and wound the throttle to maximum in an attempt to regain rotor RPM. In doing so, he oversped the engine and rotor, and subsequently overshot the landing site. He was convinced that he had a genuine power problem and elected to land in a gully beyond the knoll. The tail rotor struck ground in the steeply sloping gully, resulting in the tailboom being cut off by the main rotor. The cabin came to rest on a heap of storm debris. During later investigation, the engine was ground run successfully. A small fuel leak existed at the carburettor but this did not affect the fuel supply to the engine. The pilot had not received any training in ridge line operations. He had made his approach using visual cues only, which can result in a steep final approach accompanied by a high rate of closure. In addition, the pilot was unaware of the local wind velocity, and the aircraft was probably subject to local airflow effects. While attempting to compensate for these, it is probable that the pilot over-pitched the main rotor blades leading to a temporary reduction in main rotor RPM. He subsequently misread the symptoms as an engine malfunction.