The helicopter had just finished being used to survey cattle and was returning for a landing. The pilot reported that the engine governor was turned on and that during a steep turn downwind, he allowed the helicopter's airspeed to decrease to approximately 20 kts. As he lowered the helicopter's nose to accelerate downwind, the Low Rotor RPM warning horn sounded indicating that the main rotor RPM had decreased below 96%. The pilot reported that he immediately lowered the collective, fully opened the throttle and further lowered the helicopter's nose to recover airspeed. As the helicopter approached the ground, he selected a decelerative attitude, a manoeuvre that would normally recover some main rotor RPM. However, in this case, the main rotor RPM did not recover. When the helicopter had descended to approximately 3 ft AGL with an airpseed of approximately 55 kts, the pilot placed it in a sideways quick-stop manoeuvre. This was done to decrease the helicopter's air and ground speed before it contacted the ground. The pilot reported that he then levelled the helicopter before running it onto a sand ridge. Despite these actions, the Low Rotor RPM warning horn continued to sound throughout the descent and landing. After sliding along the ground for approximately 20 ft, the helicopter somersaulted. After the dust had settled following the crash, the pilot reported that he checked the welfare of his passenger and then checked the engine. The pilot reported that the engine exhaust was cool enough to hold and that the rocker cover was only warm to touch.