The helicopter flight instructor stated that he was providing the student pilot with his first introductory flight in a helicopter. The flight instructor was holding the dual controls lightly while the student pilot practiced hovering. The helicopter was hovering over a concrete area on the airport and had drifted over grass. The flight instructor then told the student pilot to hover back over the concrete. As the helicopter was proceeding back to the concrete, the student pilot made an abrupt movement on the cyclic to the right and the right skid contacted an unknown object, causing a dynamic rollover. The student pilot's written account was similar, except he stated that the flight instructor had taken control of the helicopter and was hovering toward the concrete when a sudden downward push of wind caused a forceful movement to the right. Examination of the wreckage by a Federal Aviation Administration inspector revealed that the right skid had contacted the ground, resulting in a dynamic rollover and substantial damage to the main rotor blades and fuselage. The inspector did not observe any preimpact mechanical malfunctions, nor did the pilots report any. The wind at the airport was recorded as calm about the time of the accident.