The juice has barely begun to ferment when the spirit is added to kill the yeast and add just enough alcohol (up to about 17%) to ensure that the natural sugar in the grapes remains and won't re-ferment.
Don't go above 38[degrees]F, or some of the super yeasts may grow and re-ferment. Most of the yeast is killed with the quick chill, but some super yeasts will still be alive.