Soluble fiber diets promote eating a large quantity of soluble fiber foods(e.g. wheat, rice, potatoes) with any insoluble fiber foods you eat as well(vegetables, meats). You also need to avoid trigger foods as well. The theory is that insoluble fiber is a trigger to the gut and eating it alone w/o any soluble fiber “padding”, will cause digestive upset. I have noticed this from time to time myself as leafy salads were never really my friend(it didn’t help I’d douse them with dressing).
You may notice that this diet is the opposite of what someone on a celiac/wheat free diet should be eating. So it’s important to take that into consideration. The best source of information about this diet is in two books: The First Year: IBS & Eating For IBS