Line, that is correct, IF makes you go and SF stops it from killing you This is how I understand the rest.
Sweet potatoes are supposed to have more SF than regular ones, right. That means that they have less starch. Starch constipates usually. For example, if you ahve diarhea, you are recommended to stick to a diet of only potatoes (regular, or white rice) and rice milk (also constipating) for awhile. I don't think the SF in the sweet potato will help you evacuate, but it would help C by giving you some food that looks like/feels starch, but is not and that helps your stool become softer and more prone to get out.
If you eat sweet potatoes that just gives you more of a chance to eat some healthy IF (because they contain more SF) which would in turn help you go.