I believe this is the process taking place when the heat pump is in the heating mode. As OSA temperatures go down, the superheat back to the compressor goes down this in turn results in a lower discharge temperature to the indoor coil so by adding heat to the accumulator it raises the superheat to the compressor which raises the discharge temperature to the indoor coil. all this raises the totat heat of the liquid to the outdoor coil which has an aparant effect on total heat of the saturation temperature. Remember the adiabatic effect of the expansion process, total heat in ..total heat out.