The function of the outdoor coil during the heating cycle is to act as the evaporator coil, which is the heat absorber. The outdoor coil absorbs heat from the outside air and transfers it to the refrigerant, which then flows to the indoor coil and releases the heat to the indoor air. This is how a heat pump can provide heating even when the outdoor temperature is cold. The outdoor coil is also where the defrost cycle occurs, which removes any frost or ice that may form on the coil surface due to the low temperature and humidity of the outdoor air. The defrost cycle helps to maintain the efficiency and performance of the heat pump.