The flight instructor reported that the pilot receiving instruction was practicing an offsite landing in an open area. While descending on approach to the site, the airspeed decayed to 35 kts. The flight instructor prompted for an increase in airspeed, the pilot flying acknowledged, applied forward cyclic, right pedal, and continued turning left into the wind. They encountered an uncommanded left yaw and the pilot receiving instruction added right pedal until he reached full deflection. The flight instructor then assumed control of the helicopter and pushed forward on the cyclic as he lowered the collective. The instructor attempted to recover from the uncommanded left yaw; however, but the helicopter spun several times. Unable to gain forward airspeed, the flight instructor began a descent. The helicopter subsequently landed hard substantially damaging the underside of the fuselage. The flight instructor reported that there were no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation.