While flying at a slow forward speed approximately 15 feet above the trees, the helicopter suddenly yawed to the right. As the pilot was unable to correct the yaw he attempted to manoeuvre the helicopter to a clear area. The helicopter impacted the ground in a level attitude, heading rearward and rotating. Investigation revealed that all the teeth were missing from the forward short shaft rear coupling, which had failed due to lack of lubrication, and resulted in a loss of tail rotor propulsion. The coupling, which was covered by a rubber dust boot, had not received adequate daily inspections to ensure that it contained sufficient grease and was free of excessive play.