On May 18, 1999, about 1730 hours Pacific daylight time, a Robinson R22A, N8558N, sustained substantial damage during a hard landing and rollover near San Luis Obispo, California. A certified flight instructor and student, the sole occupants, were practicing autorotations in a hilly area approximately 5 miles east of the San Luis Obispo airport. Both pilots received minor injuries. Helipro, Inc., of San Luis Obispo, was operating the instructional flight under the provisions of 14 CFR Part 91. The flight originated in San Luis Obispo about 1630 and no flight plan was filed. Visual meteorological conditions prevailed. An aviation routine weather report (METAR) issued for San Luis Obispo at 1717 indicated broken skies at 1,000 feet with winds at 17 knots gusting to 24 knots. The operator stated the pilots received a DUATS weather briefing prior to departure. They departed to the east, which was clear but windy. The instructor stated the student entered a practice autorotation at 1,400 feet (600 feet agl). The selected landing site was on top of a hill approximately 800 feet high. As the flare was initiated at 40 to 50 feet, the instructor mentioned to the student that the cyclic trim was engaged. When the student unexpectedly reached forward and tried to push the trim control knob in, he inadvertently pushed forward on the cyclic control. The instructor said this pitched the nose of the helicopter down and resulted in a bleed off of main rotor rpm. When the low rotor horn and warning light came on the instructor took control of the helicopter and rolled in throttle. The lowest rpm he recalls seeing was slightly above 90 percent. He made a left turn to avoid hitting the hill and encountered a strong downdraft due to the gusty winds. The helicopter touched down on the left skid during the turn and rolled over twice. The pilots were able to extricate themselves and walk to a farmhouse for help. The instructor stated the cyclic trim was used during cruise to relieve stick loads. He indicated the helicopter could be easily controlled in all flight regimes with the cyclic trim on or off. He normally flies with it off, but teaches its use because the flight examiner required a demonstration of its use on check rides. He said there was no checklist requiring it be turned off prior to practicing autorotations. He said it was controlled by a push/pull knob located on the console in front of the cyclic. The knob has to be pushed in approximately 4 inches to turn the trim off.