The helicopter flight instructor reported that the commercial pilot rated/helicopter flight instructor candidate receiving instruction was practicing autorotations with power recovery to the runway; during a practice 180 degree autorotation, the flight instructor candidate executed the landing flare too low to the ground resulting in a tail rotor strike. The flight instructor further reported that he took the controls and landed the helicopter after it had yawed, un-commanded to the right, about 180 degrees. The helicopter sustained substantial damage to the vertical stabilizer and tail rotor drive assembly. The flight instructor reported that there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation. The FAA Helicopter Flying Handbook FAA-H-8083-21A (2012) discusses procedures for autorotation and states in part: Care must be taken in the execution of the flare so that the cyclic control is neither moved rearward so abruptly that it causes the helicopter to climb nor moved so slowly that it does not arrest the descent, which may allow the helicopter to settle so rapidly that the tail rotor strikes the ground… extreme caution should be used to avoid an excessive nose high and tail low attitude below 10 feet. The helicopter must be close to the landing attitude to keep the tail rotor from contacting the surface.