The certified flight instructor reported that he took off in the helicopter to show a student pilot a demonstration of various maneuvers. Near the end of the flight the instructor demonstrated a power on autorotation but he was late rolling on the throttle and did not have sufficient rotor RPM to recover the helicopter before impacting the ground. Upon ground impact the helicopter's tail rotor drive shaft and main rotor blades were damaged. The pilot reported no pre-impact mechanical malfunctions of failures with the airframe or the engine that would have precluded normal operation.