The pilot, who did not hold an instructor rating, was conducting a trial instructional flight. Prior to DEPARTURE, he had briefed the passenger on the functions of the various controls in the helicopter. On the downwind leg of the circuit, the passenger was allowed to handle some of the controls. The aircraft was subsequently placed in a hover at about five feet above the ground, and the passenger was invited to attempt to control the helicopter by use of the anti-torque pedals and the cyclic control. The passenger overcontrolled the aircraft, and the pilot was unable to prevent it from striking the ground in a steep nose-down attitude. Following the impact a fire broke out and destroyed the aircraft. No mechanical fault was subsequently discovered which might have led to the development of the accident. The pilot had undertaken the flight because no qualified instructor had been available at the time the passenger arrived, and he wished to avoid potential embarrassment to the company.