THE FLIGHT INSTRUCTOR (CFI) REPORTED HE AND THE STUDENT PILOT WERE PRACTICING SIMULATED EMERGENCY AUTOROTATIONS TO A POWER RECOVERY. DURING THE ACCIDENT AUTOROTATION, THE CFI SAID HE ADJUSTED THE POWER TO 3100 RPM AND THEN TOLD THE STUDENT TO RECOVER. THE CFI NOTICED THAT THE HELICOPTER'S DESCENT HAD NOT BEEN ARRESTED, AND HE JOINED THE STUDENT ON THE CONTROLS. THE CFI WAS ATTEMPTING TO AUTOROTATE WHEN THE HELICOPTER LANDED HARD. POSTACCIDENT INSPECTION OF THE HELICOPTER AND ITS ENGINE DISCOVERED NO EVIDENCE OF PREIMPACT MECHANICAL ANOMALIES.