The flight instructor and pilot receiving instruction were practicing high density altitude training. The pilot receiving instruction made several landings to a pinnacle approved for practice landings by the flight school which was located at about 5,500’ ft msl. The flight instructor decided to try a new landing site, which was not approved by the flight school, located at 6,500’ ft msl. On their second approach to the new landing area the rotor RPM began to decay, the flight instructor took the controls, he pushed forward on the cyclic to attempt to go around and added power, increasing collective. The instructor’s control inputs resulted in the main rotor rpm’s continued decrease. The helicopter impacted the ground resulting in substantial damage to the fuselage and tail boom. The pilot reported no pre-impact mechanical malfunctions or failures with the helicopter that would have precluded normal operation.