The acetic acid in the apple cider vinegar interacts with the small intestines reducing the uptake of simple carbs and aids in the transition of running on fat rather than carbs. Hence the insulin spike is reduced when ingesting carbs. I suspect that you are getting a side benefit of disrupting the production of stomach acid in response to ghrelin production. Ghrelin production is a partially learnt response and comes in waves shortly before the intake of food. Suppression of ghrelin means less stomach acid production, hence less heartburn. A bonus.