According to the certificated flight instructor, he was performing hover practice with the student pilot in a grassy area beside a taxiway. During the practice, the helicopter climbed to approximately 20 feet above ground level, and then the instructor advised the student to position the helicopter at a lower altitude. The instructor stated that the helicopter began “a high rate of descent which [he] was unable to arrest prior to ground contact.” He also said that the helicopter developed a “dynamic rolling moment that full down collective was unable to stop.” The helicopter rolled and came to rest on its left side, substantially damaging the tail boom, skids, and main rotor. The winds reported at the airport about the time of the accident were from 250 degrees at 13 knots gusting to 17 knots. The pilot did not report any mechanical anomalies with the helicopter.