The student helicopter pilot was at an early stage of his training, however, he was a very experienced fixed-wing pilot. Before conducting some circuits he was given practice in hovering. After this the instructor took over to demonstrate the manoeuvrability of the helicopter with the rotor disc steady. During this hovering the height increased to about 30 feet before the aircraft began to spin rapidly in an anti-clockwise direction and dive towards the ground. The low rotor rpm warnings sounded during the descent. Impact evidence showed that the helicopter had struck the ground at a high rate of descent while travelling forward slowly. No signs of engine malfunction were found.