During a descent for an off airport pinnacle landing, the helicopter encountered a downdraft about 75 to 100 feet above ground level (agl), and the pilot receiving instruction, increased collective. The flight instructor instructed the pilot receiving instruction to be less aggressive using the collective as the flight continued the descent to the landing area. As the helicopter descended through about 10 to 25 feet agl, it encountered a second downdraft and the pilot receiving instruction increased collective followed by a decrease in rotor RPM. Subsequently, the flight instructor took control of the helicopter as it contacted the ground and then became airborne again. The helicopter yawned to the right, and landed upright on sloping terrain. Post flight examination of the helicopter revealed that the tailboom was structurally damaged just aft of the fuselage attach point. The pilots reported no preimpact mechanical malfunctions or failures with the helicopter that would have precluded normal operation.