Why does the ac compressor turn on and off when the defroster is on?

Most automotive ac systems will turn on the ac anytime the HVAC (heating, ventilation and air conditioning) controls are set on defrost even when the temp control is set on heat. By doing so the moisture in the air is removed and it will clear a foggy inside windshield quickly. As to the ac compressor turning on and off continuously, this could be a sign of being low on freon.