The pilot reported that he was conducting annual refresher training with a certified flight instructor. During a practice autorotation to runway 18, the helicopter descended below a nearby tree line and "dropped vertically." He stated that the helicopter descended so rapidly that he was unable to arrest the descent rate and the helicopter landed hard. The pilot reported the winds from 230 degrees at 10 knots with gusts to 19 knots at the time of the accident. He reported that, as the helicopter descended below treetop level, "…we lost all lift and wind advantage…" The hard landing resulted in a fractured tail boom. The pilots reported no mechanical malfunctions or failures that would have precluded normal operation.