The PTC relay is formed by a ceramic material insert

This material has the property of increasing electrical resistance when heated by the current that passes through it. During the engine start, the PTC is cold, and with a low electrical resistance, consequently, it conducts current through the starting coil, causing the engine to rotate. This current will heat it up causing the resistance to increase and the current to decrease through the starting coil until it becomes practically zero.
Its use is recommended for freezers and domestic refrigerators, where the time between operating cycles is sufficient for the PTC to cool down and be ready for a new start.