This is something that is always up for debate at our group, the way to understand it is, Slimming World advocate 'unlimited foods' this gives you natural portion control at meal times. As soon as you change the composite of the food you will eat more in it's changed form. Our Leader has always said if your going to have a bag of crisps, have one, and syn them. I always say anything in moderation if it's working for you then don't do it this way.
This is how I work it in for example; when I have a salad I sometimes leave out the pasta that I would normally mix through it, but then part cook and bake 100 grams (dry) mix with marmite, this gives me the snack fix that I want now and then and it's something I would have included in my meal anyway. Thus not increasing my carb intake by snacking on top of eating the carb in with my lunch.