I may have misunderstood, or there may be variations, but: I understand the portion of sour mash to be, just as in German brewing (except for its being carried forward), a pH control for the mash (added at the beginning of mashing,) which may then be fermented by a house culture yeast (some like Jack Daniel's call it jug yeast) which may in turn be recultured and repitched. So multiple levels of throughput. I don't suppose you'll get the full story on anybody's tour! (Well, I've learned the secrets at the local guys, but they aren't too worried about espionage at their scale!)