During a flight for the purpose of the pilot to acquire the minimum flight hours to act as pilot-in-command, he and a pilot providing training (who was not a flight instructor) of the helicopter were practicing autorotation's from flight to a power recovery. The pilot providing training stated that on the fourth autorotation the pilot allowed the rotor rpm to decay and then froze on the controls, not allowing him to increase the throttle. He was able to level the helicopter; however, it landed hard and the main rotor blades struck and severed the tail boom. The pilot reported that at the initiation of the maneuver, the pilot providing training rolled the throttle off, and the downwind glide was established. The pilot stated that as he leveled the helicopter and collective was pulled, he realized that the engine was not completely back on line. The pilot providing training subsequently took over the flight controls and flew the helicopter to the ground. Both pilots reported the need for better verbal commands or stating intentions throughout the maneuver. Neither pilot reported a preimpact mechanical malfunction or failure with the helicopter that would have precluded normal operation.