The operator reported that a hard landing occurred during a night training flight that was conducted for a newly hired helicopter pilot and a company flight instructor candidate. The training was being conducted by the flight instructor candidate while both pilots were being observed by a company flight instructor from the right rear seat. After conducting numerous other training events during the flight, the flight instructor announced to the pilot-under-instruction that the next event was going to be a straight in autorotation from altitude with a power recovery. The instructor then briefed the pilot-under-instruction on how the maneuver was to be conducted. During the descent the pilot-under-instruction initiated deceleration using aft cyclic. The instructor began increasing the throttle to the maximum position. However, the instructor noticed that the rpm was about 95% and he believed the engine had not responded appropriately. He then responded by moving the throttle towards the idle position. Recognizing his mistake, he then attempted to correct by increasing the throttle back to maximum power. After this he focused his attention outside the cockpit due to the low altitude. Shortly after the pilot-under-instruction recalled hearing the instructor say something to the effect, it’s not recoverable, prepare for impact. He attempted to arrest the decent before impact but was unsuccessful. The helicopter subsequently impacted the runway and came to rest on the lower fuselage which resulted in substantial damage to the tail boom. The operator reported that there were no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation.