To put it simply, they don't work. You can't 'block' fat or carbohydrates from being absorbed.
What you can do, is increase muscle insulin sensitivity to help partition the food you eat into muscle cells and away from fat cells. Look into the product n2slin from n2bm.com.
N2slin is a non-stimulant fat burner and powerful nutrient partitioner. It increases insulin sensitivity of muscle cells and helps partition the food you eat into muscle cells and away from fat cells.
Take 2 tablets N2slin 30 minutes before each meal you eat that contains a significant amount of carbohydrates. If you are having a particularly large meal (like a cheat meal), use 2 pills 30 minutes before your meal and 2 pills 30 minutes after.