On October 25, 1999, at 1720 eastern daylight time, a Robinson R-22-Mariner, N2285Z, landed hard during a practice autorotation landing in West Boca Raton, Florida. The helicopter was operated by the certified flight instructor under the provisions of Title 14 CFR Part 91, and visual flight rules. Visual meteorological conditions prevailed and no flight plan was filed for the local instructional flight. The instructor received minor injuries, the private pilot rated student was not injured, and the helicopter sustained substantial damage. The flight originated from the Ft. Lauderdale Executive Airport, Ft. Lauderdale, Florida, at 1600. According to the instructor, he and his student were practicing 180-degree autorotation landings with power recovery. They began practicing autoroations to the right, then switched to the left. Prior to initiating their last autorotation to the left, they climbed to 500 feet above ground level (agl). The student was at the controls and began a steeper turn in an attempt to reach the aiming spot. The helicopter then entered a "slight dive." As the student rolled out to begin the flare, the helicopter was below 100 feet agl. According to the instructor, he took the controls and pushed forward to level the helicopter. He rolled the throttle and tried to pull up on the collective. The helicopter hit the ground in a level attitude, then became airborne again. During the impact sequence, the tail boom was severed by the main rotor system, and the helicopter became uncontrollable. The helicopter then spun three or four times and landed on its left side. The instructor stated on his accident report that there was no mechanical malfunction with the helicopter.