On April 14, 2004, at approximately 0930 mountain daylight time, a Robinson R22, N555UH, was substantially damaged during a hard landing at Nephi, Utah. The flight instructor and pilot receiving instruction were not injured. Visual meteorological conditions prevailed. No flight plan had been filed for the flight being conducted under the provisions of Title 14 CFR Part 91. The cross-country flight had departed Provo, Utah, at 0800. According to the flight instructor, they were practicing 180 degree autorotations and had just successfully completed one. Prior to the second maneuver, the pre-landing checks were completed and the maneuver was entered at an altitude of 700 feet agl and at an airspeed of 75 knots. The flight instructor stated "at approximately 150 feet agl, [the] decent rate increase[d] dramatically." He said that he attempted to recover and flare with "aft cyclic and down collective" with no effect. He added full throttle and "raised" collective, in attempt to land in a "skid level attitude." During the impact with the ground, both skids collapsed and the main rotor blade severed the tail boom. The helicopter rotated "three or four times" before coming to rest off of the left side of the runway. An examination of the systems revealed no anomalies.