While performing simulated power-off autorotations with a power recovery, the certified flight instructor (CFI) and student noted that the main rotor rpm was decaying. About 150 to 200 feet above ground level (agl), the CFI took the flight controls and executed a flare about 50 feet agl, and realized the helicopter was going to settle. The CFI then attempted a run on landing, but the helicopter landed hard, spreading the skid landing gear. The helicopter then slid about 75 feet down the runway, departed the runway surface, and rolled four times before coming to rest on its left side. The CFI stated that there were no mechanical problems with the helicopter.