The pilot under training had recently completed a Private Pilot Licence course on Robinson R22 helicopters. This was the first flight of his conversion to the AS350-B type. After about ten minutes hovering practice the pilot carried out a successful landing. He was instructed to come to the hover again and perform another landing. The instructor stated that on touchdown the pilot made a sudden movement of the cyclic control, causing the tail rotor to strike the ground and breaking both blades. The helicopter swung to the left through 180 degrees and one landing skid was broken. The helicopter type has light and sensitive controls. The pilot had probably applied excessive cyclic inputs as the helicopter touched down. The instructor was unable to take control in time to prevent the tail rotor strike.