It's not strictly true that if you eat less than 5% fat, the tablets don't do anything.
If you take the tablets as normal (at each mealtime) they will always get rid of a third of the fat in that meal, regardless of whether it's a low fat or a high fat meal.
For exmple, if you eat a meal with 9g of fat (well within the 15g per meal rule), the tablet will make 3g of that fat pass straight through your body via your stools (but you probably won't know about it because the amount is so small).
You only encounter problems when you eat foods with high fat contents, because the quantity of fat being passed out in your stools is more.
For example, if you ate a meal with 60g of fat in it, 20g of it would go straight through your body, and you would definitely know about it!!!
When you take these tablets as prescribed, and stick to the 5g per 100g of food rule (and no more than 15g per meal), you probably won't see anything happening, other than maybe a bit of bloatedness or constipation (as Red described), but you will notice the difference on the scales - partly because the tablets are getting rid of some of the fat you're eating, but partly because you'll eat more heathily because of the threat of the tango effect if you don't!!!!