Fermenting beer is added to finished beer at packaging. In the right amount,it will give the proper carbonation level and the yeast can also clean up any fermentation by products. A big production brewery always has plenty of fermenting beer on hand to prime with, if they are set up for that process.
Kai covers it on his site, and if you google Krauesening Calculators you will find those on the net.
People often confuse krauesing with the addition of speise, which is just adding wort that is not ferementing.