The flight instructor in the helicopter reported that, while demonstrating a touchdown autorotation to the pilot receiving instruction, the helicopter landed in a "normal run-on landing configuration." He added that the pilot receiving instruction applied aft cyclic about 15 ft into the forward ground run. The flight instructor moved the cyclic into the neutral position and lowered the collective as quickly as possible, to slow the ground speed. The helicopter "jumped" up and yawed to the left and spun the helicopter about 150° before coming to rest. The flight instructor felt abnormal vibration in the cyclic and collective. He opened the door to inspect the helicopter and saw that the main rotor had struck the tailboom. The helicopter sustained substantial damage to the tailboom. The flight instructor reported that there were no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation. The automated weather observation station located 12 miles north of the accident airport reported that, about 17 minutes before the accident, the wind was from 220° at 7 knots. The helicopter landed on runway 18. Federal Aviation Administration's Helicopter Flying Handbook, FAA-H-8083-21A, contains a section titled "Autorotation " which states: 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.