The single engine helicopter impacted the ground hard during an autorotational landing following a simulated loss of engine power. The flight instructor was simulating the loss of engine power to his student and noticed the rotor rpm was low when the helicopter was approximately 400 feet above the ground. The instructor ensured the collective was lowered all the way and placed the cyclic forward, but was unable to recover the rotor rpm. He flared the helicopter, but it landed hard resulting in the main rotor blades severing the tail boom.