During an instructional flight, the certified flight instructor (CFI) was demonstrating a 180-degree autorotation for a pilot rated student. The CFI positioned the helicopter on a downwind, abeam the landing site, about 800 feet above ground and at about 70 knots when he started a right turning autorotation. After turning about 80 degrees the pilot rated student told the CFI that it appeared they were going to land short on the approach. The CFI increased the helicopter's turn rate; however, near the end of the turn, he noticed the rotor RPM was starting to decay and he applied down collective. As the helicopter was rolling out of the turn, it appeared that the helicopter was descending faster than moving forward. The CFI began the cyclic flare to reduce the descent; however, the helicopter impacted the ground hard and rolled onto its left side. The helicopter sustained substantial damage to the tailboom and fuselage structure. The CFI reported there were no mechanical malfunctions or failures that would have precluded normal operations.