Controls also have the ability to cool a part down slowly. One option is downslope described in another article. Post heat is also a means to accomplish this goal. In post heat the current is reduced at the end of the weld cycle to a level less than the weld current for a period of time before initiating the hold and off portion of the weld sequence.
Post heat might be used to prevent or reduce quenching a part. This could be for metallurgical or distortion reasons. It may also temper the resultant weld affected zone. It could also be to extend the molten nugget time for growth but at a slightly lower temperature.
Reference: RWMA - Resistance Welding Manual 4th Edition