The flight instructor reported that, during the instructional flight and while conducting a maximum-glide configuration autorotation, he and the student pilot set the rotor rpm between about 93% and 94%. The instructor began applying aft cyclic to slow the descent rate and to regain rpm; however, the descent rate did not decrease, so he applied more left cyclic to no avail and then increased the throttle. The helicopter landed hard at the most level attitude the instructor could make it, and it then bounced and came to rest. The tailboom was substantially damaged. The Pilot's Operating Manual contained a caution that stated to "increase rotor RPM to 97% minimum when autorotating below 500 ft above ground level." The chief flight instructor reported that there were no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation.