The Bell 206B was being flown on a helicopter rating ride with the PIC in the right seat and an FAA inspector in the left seat. The oral was satisfactory and the flight check progressed to autorotations. The first one was long, the second short. On the third one, which was also short, the FAA inspector commanded a go-around. The helicopter continued to descend, and the FAA inspector tried to roll on the throttle but it would not move. A low rotor RPM situation developed and the helicopter made a hard landing with the tailboom being struck by the main rotor blades. The PIC said he rolled on the throttle. The FAA inspector secured the engine after the accident. He said he remembered the throttle being at flight idle when he shut it down. Examination of the aircraft showed that the throttle could be held with a tight grip on one side making it difficult to be moved from the other side. No evidence of an engine or throttle malfunction was found.