On May 1, 2000, about 1645 hours Pacific daylight time, a Hughes 269C, N5395S, operated by Western Operations, Inc., landed hard at the (uncontrolled) Rialto Municipal Airport, Rialto, California. The helicopter was substantially damaged, and the commercial certificated flight instructor (CFI) and the student sustained minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed. The instructional flight was performed under 14 CFR Part 91, and it originated from Rialto about 1600. The CFI indicated to the National Transportation Safety Board investigator that he was providing dual instruction to the student, and the student was handling all of the engine and flight controls. The CFI was providing training to the student who was a CFI candidate. The accident occurred during a practice, 180-degree full touchdown autorotation, when the helicopter's airspeed decreased and the main rotor rpm became too low. The CFI indicated that he observed the low rpm indication and called it to the attention of his student, but the student did not take decisive corrective action by promptly initiating a power recovery to avoid the resultant low rotor rpm flare and hard touchdown. No mechanical malfunctions were noted. The student reported to the Safety Board investigator that he holds an airline transport pilot certificate with both airplane and rotorcraft ratings. He also holds a CFI certificate for airplanes. The purpose of the flight was to receive instruction in furtherance of a rotorcraft instructor's certificate. The student indicated that he commenced the practice full touchdown autorotation from 2,000 feet mean sea level (about 545 above ground level), and abeam the desired taxiway touchdown point. He stated that, "As I rolled out on final at 300 feet (agl) I was low." The student indicated that although he had initiated a power recovery, the helicopter impacted the ground hard. He also stated that no mechanical malfunctions occurred during the flight.