Ford Motor Co. Cold Start Spark Hold System: Operation

When engine coolant is less than 128°F, the CSSH PVS is closed and the distributor vacuum signal travels through the restrictor.

At cold start, high vacuum acts on the distributor diaphragm, giving maximum spark advance. During cold acceleration, the high vacuum already in the distributor diaphragm is slowly bled down through the restrictor, providing a modified vacuum advance during the initial stage of acceleration.