It boosts the growth/multiplication of the yeast. Kind of like steroid drugs would. You could say that caffeine “feeds” the yeast. Bummer to avoid all kinds of yummy food trying to starve yeast just to turn around and feed it again with coffee, you know? That’s why it’s not allowed on the diet. I had to wean myself gradually even though I only drank one cup of coffee a day. I hadn’t been completely caffeine free for a while, so I decreased my daily amount until I was totally off it around day 10.