When an outdoor condensing unit is installed in an area of the country where the outdoor temperature could drop lower than the evaporator’s temperature, it will generally use a pump-down system to control the operation of the compressor. This design will prevent refrigerant from migrating back to the compressor during the off cycle, when the compressor’s temperature is less than that of the evaporator.
Refrigerant migration can be damaging to a compressor. The refrigerant migrating back to the compressor will likely condense into a liquid, and on startup, the compressor will start with liquid refrigerant inside, which will lead to bearing washout and possibly slugging the compressor.