The student pilot reported that while practicing 180 degree autorotations, about 500 feet above ground level (agl), the flight instructor reduced the throttle and the student pilot lowered the collective while simultaneously applying aft cyclic and right peddle. He further reported that the helicopter began to "dive" and the flight instructor took control of the helicopter. The flight instructor reported that he initiated a go-around by applying aft cyclic and raising the collective, but they "continued in a rapid descent." The flight instructor further reported that "roughly fifty feet [he] flared harder to attempt a next-to-final arrest". The helicopter landed hard, the skids collapsed, and the main rotor blades severed the tailboom. The helicopter slid about 300 feet down the runway toward the right; the right skid stuck into the grass and the helicopter rotated clockwise about the right skid, tipped onto the nose, and rolled to the left. The flight instructor reported that he and the student pilot had performed about 10-12 practice autorotations prior to the accident. The helicopter sustained substantial damage to the fuselage. The flight instructor reported that there were no preaccident mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation. Robinson Helicopter Company published a safety notice, SN-38, dated July 2003, and revised in October 2004, addresses training accidents caused by practice autorotations. It states, in part: A high percentage of training accidents occur after many consecutive autorotations. To maintain instructor focus and minimize student fatigue, limit practice to no more than 3 or 4 consecutive autorotations. The Federal Aviation Administration (FAA) published the Helicopter Flying Handbook, FAA-H-8083-21A, dated 2012, addresses tailboom strikes during autorotations. The handbook states, in part: A power recovery can be made during training in lieu of a full touchdown landing. Refer to the section on power recovery for the correct technique. After the helicopter has come to a complete stop after touchdown, lower the collective pitch to the full-down position. Do not try to stop the forward ground run with aft cyclic, as the main rotor blades can strike the tail boom. Rather, by lowering the collective slightly during the ground run, more weight is placed on the undercarriage, slowing the helicopter. One common error is holding the helicopter off the surface versus cushioning the helicopter on to the surface during an autorotation. Holding the helicopter in the air by using all of the rotor rpm potential energy usually causes the helicopter to have a hard landing, which results in the blades flexing down and contacting the tail boom. The rotor rpm should be used to cushion the helicopter on to the surface for a controlled, smooth landing instead of allowing the helicopter to drop the last few inches.