The pilot brought the helicopter to a hover at 1000 feet agl, pointing approximately into wind. The aircraft began to yaw to the right and the pilot was unable to stop the resulting rotation. The helicopter descended in a steep nose down attitude and struck the ground heavily while still rotating to the right. The landing skids were torn off and the helicopter came to rest on its left side. No mechanical fault or defect was found with the helicopter which might have contributed to the development of the accident. It was considered likely that the aircraft experienced the phenomenon known as "tail rotor breakaway", which results in an uncommanded yaw to the right accompanied by a steep nosedown pitch change. The pilot was aware of the phenomenon, and had read various articles on the subject. However, much of the information available at the time was of a confusing and conflicting nature, and the recovery action employed by the pilot on this occasion was ineffective.