The certified flight instructor (CFI) reported that, after takeoff, the student pilot practiced hovering maneuvers then practiced a straight-in autorotation with a planned power recovery. The student pilot initiated the autorotation between 300 and 400 feet above the ground with a 5- to 6-knot tailwind. During the descent, the helicopter descended faster than anticipated, and, although the student pilot initiated a flare to arrest the descent, by that point, the helicopter was too low and still descending too fast. Just before the skids hit a mound of dirt and rock that was about 6- to 8-feet tall, the CFI took the controls but was unable to arrest the descent. The CFI did not apply power but attempted to back off the mound and turn around. Because the engine was at idle power and the rotor rpm was decreasing, the helicopter spun and came to rest on its left side. The CFI reported that there was no preimpact mechanic failure or malfunction. The helicopter sustained substantial damage to the tailboom and also to the right side of the helicopter in the area of the right aft skid.