The task of the expansion valve is to dispense enough refrigerant to evaporate in the evaporator. He does this by changing his throat. Reduced cooling performance or compressor consumes fewer refrigerant vapors, expansion of the expansion valve increases. Thus, you add less refrigerant to the evaporator and vice versa. If the expansion valve works well, it works to keep the evaporator always as much but only enough fluid-based refrigerant to evaporate there. The refrigerant leaving the evaporator should be in a steamy state to prevent liquid from entering the compressor.