During a flight review the private pilot completed three practice autorotations to an unimproved open field, terminating in a power recovery. During the forth practice autorotation the pilot could not get the engine to respond during the power recovery. The flight instructor took over the controls, at about 100 feet above ground level, and completed the autorotation to a touchdown. According to the flight instructor, he had to flare steeply to avoid a tree line. The helicopter touched down on the heels of the skids and tail stinger. The aft section of the tail boom separated from the helicopter when the main rotor blades contacted the tail boom and the tail rotor assembly. The pilot said the flight instructor initiated an aggressive flare and struck the ground hard, and that there was a violent shudder as the helicopter leveled the skids. The helicopter came to rest upright. There was no post crash fire. The pilot reported a minor injury and the flight instructor reported no injuries. An examination of the helicopter's engine by Federal Aviation Administration (FAA) inspectors did not reveal any pre-impact anomalies; additionally they reported that more than a half tank of fuel remained in the helicopter at the accident site.