The pilot reported that, while hover-taxying the helicopter prior to commencing a circuit, it suddenly yawed sharply to the left. He applied right pedal and initially lowered the collective pitch control before raising it. However, the helicopter landed heavily, damaging the front and rear crosstubes. No fault was found with the helicopter which might have contributed to the accident. The pilot reported that he was not sure that an engine failure had occurred but that he reacted instinctively when the helicopter yawed left. Discussion revealed that the pilot rode a motor cycle, the hand throttle of which operated in the reverse sense to that on the helicopter. It is possible that the pilot inadvertently reduced the throttle setting, thus inducing the yaw. His limited flying experience and exposure to engine failure situations then contributed to him lowering the collective pitch control as part of the initial reaction to the yaw. The helicopter was at too low a height to prevent a heavy landing from the descent caused by the lowering of the collective pitch control.