A certified flight instructor (CFI) and a student were practicing 180-degree autorotations. The instructor noted that during the initial entry into the autorotation, the student pilot maintained proper rotor rpm and airspeed. About halfway through the 180-degree turn, the helicopter "became nose low" and the airspeed increased. As the helicopter transitioned through about three-quarters of the turn, the instructor took control of the helicopter and attempted to correct the airspeed. The instructor stated that she was unable to correct and the helicopter touched down hard in an open dirt field. The helicopter subsequently rolled over and came to rest on its left side. Examination of the helicopter revealed that the tail boom was separated and the fuselage sustained structural damage. The flight instructor reported that no mechanical malfunctions existed during the practice autorotation and accident sequence.