From my understanding sour mash is reusing the yeast, At one time they did not have yeast as we do now .so reusing yeast made it more consistent.
also they did not "cook" the mash so there was a lot of starch left in the "back set" . putting the mash in the still and "cooked" it bringing
out the starch, then using that for the next mash.
so two things happened
1) they reused the yeast
2) they cooked the grain in the still. (at the same time taking off the alcohol that was there)