The instructor reported that during a training flight, while practicing an autorotation, the student began the cyclic flare and the tail of the rotorcraft struck the runway. The power recovery was initiated by the student and the instructor began to assist on the controls. The instructor reported that there was very little tail rotor effectiveness and that the aircraft then began to spin, made contact again with the ground and came to rest on its left side. The pilot reported no mechanical failure or malfunction with the helicopter prior to the accident.