Cyndy - the fundamental basis of Heather's diet is to avoid foods/food combinations/meals etc. that over-stimulate the colon, and to eat foods that soothe and regulate its movements. IF can, if not eaten carefully enough, over-stimulate the colon and cause spasms - which, depending on the person can cause D or C. (Alternatively, SF soothes the colon). IF is *definitely* important for the function of the gut overall though, but for those of us with IBS, the colon and whole GI system is just a bit more sensitive to being over-stimulated. So it's kind of a balancing act - finding the right combination of IF and SF (amounts etc.). Make sense?
(I am not C - mostly A/D - but that is almost a direct quote from Heather's EFI.)