The student had a total of 45 hours helicopter flying, and also had a Private Pilot Licence with 130 hours fixed wing experience. During a period of practice circuits and engine failures in 15 to 20 knot wind conditions, the student required several practice autorotative landings in order to reach a satifactory standard. These exercises were commenced about 700 feet above the ground. During this period, the Tower advised that the wind strength had increased to 35 knots. Conditions remained stable and the instructor elected to continue with the training. A further engine failure after take-off was simulated from about 400 feet, and on successful completion of this manoeuvre, the instructor simulated a failure at 100 feet. On this occasion a high rate of descent developed and the instructor took control. The touchdown was firm, and was on the heels of the skids. The helicopter rocked forward and the main rotor struck the tail boom. It was considered probable that the aircraft had been affected by a reduction in wind speed at the time the engine failure was simulated. The low height at which the manoeuvre was being performed did not allow sufficient time for adequate corrective action to be taken to arrest the rate of descent.