I think that the pressostat or low pressure switch has two functions. One is to protect the compressor if the refrigerant is so low that proper lubrication is not possible for the compressor. Two is to cut of the compressor when the low side loses pressure due to high rpm. At high rpm the compressor socks the refrigerant rapidly from the low side and is packed in the high side. The pressure falls below normal in the low side and the pressostat opens the circuit and cuts off the compressor. After the compressor cuts off the refrigerant is still packed in the high side and continues to pass through the expansion valve or orifice tube and builds up pressure in the low side. When pressure is normal again the pressostat closes the circuit and the compressor turns on again. At idle speed rarely you can hear the compressor cutting off unless the refrigerant is low or the engine is extremely hot. When the engine is extremely hot a high pressure switch at the condenser may come in to play and cut off the compressor if the pressure is to high due to temperature.