The purpose of the flight was for the certified flight instructor (CFI) to evaluate the skill level of the second pilot, and to provide any needed instruction. The CFI assigned the second pilot the task of performing an autorotation to a landing. During the approach the second pilot, who was handling the flight controls, was a little short of the desired touchdown mark and permitted the main rotor rpm to decrease. During the hard touchdown a rotor blade chopped off the tail boom.