The flight instructor in the helicopter reported that, while the pilot receiving instruction was practicing "slope landings" in the tall grass adjacent to an asphalt runway, he was shadowing the flight controls. He added that the pilot moved the cyclic to the left. The instructor was unable to correct, and the helicopter experienced dynamic rollover and impacted terrain. As a recommendation, the flight instructor reported that the accident could have been prevented by not practicing slope landings in tall grass. He added that the helicopter contacted the terrain sooner than expected due to the tall grass. The helicopter sustained substantial damage to the fuselage and firewall. The flight instructor reported that there were no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation. The automated weather observation station, located about 12 miles south of the accident, reported that, about 29 minutes before the accident, the wind was from 170° at 15 knots, gusting 21 knots. The same automated station reported that, about 9 minutes after the accident, the wind was from 180° at 9 knots. The pilot was landing to the south.