The aircraft crashed during an attempted practice autorotation after power was applied to effect a recovery. The CFI, on the flight, stated that he noticed that the student was going to overshoot and instructed him to make a power recovery at 100 ft AGL. He said that the aircraft shuddered and entered a high sink rate. By the time the CFI took the controls, it was noticed that the rotor RPM was below the proper range and decaying. The engine RPM was noted to be near zero. The CFI reported that he then 'bottomed' the collective and opened the throttle but the engine did not respond. In spite of the CFI's efforts to recover, the aircraft hit the ground tail low and yawed left. After sliding approximately 20 ft, the aircraft rolled over. FAA checked the engine after the rollover and found it started and ran normally with proper magneto operation and a smooth idle at 1300-1350 RPM. Manually reduced to 500-600 RPM, the low side cushion returned the engine to normal idle at throttle release. No evidence of an engine malfunction was found.