The certified flight instructor (CFI) reported that after takeoff, hovering was performed followed by a practice straight-in autorotation with a planned power recovery. The autorotation was initiated between 300 and 400 feet with a 5 to 6 knot tailwind. During the descent they descended faster than anticipated and near the end of the autorotation they “…lost all our lift and started our flare to try and slow us down, but by that point we were too low and still descending sic [to] fast.” The skids hit a mound of dirt and rock which was about 6 to 8 feet above ground level, and he did not apply power, but attempted to back off the mound and turn around. Because the engine was at idle power and decreasing rotor rpm, the helicopter spun and came to rest on its left side. The CFI further reported there was no preimpact mechanic failure or malfunction. Digital pictures provided by the Federal Aviation Administration (FAA) inspector-in-charge depict structural damage to the tailboom, and also the right side of the helicopter in the area of the right aft skid.