During a low-level photographic flight, the pilot made a shallow left turn at about 400 ft agl and a low forward speed. When he increased the rate of turn, the helicopter began a rapid turn to the right and the nose pitched up. The rotation continued for 3 to 4 turns during which the nose pitched up and down. The pilot had almost regained control when the helicopter struck the ground heavily on its skids and rolled onto its right side. The cause of the accident could not be conclusively determined but no mechanical defects were detected. The most probable cause was the operation of the helicopter in a mode that induced tail rotor stall and at a height that precluded a safe recovery following loss of control.