It's if you use food in a way it was not intended. For example, people began making 'lasagne dorito's' from baking sheets of lasagne. Whilst lasagne sheets are free on a green day, if they are baked then they are nowhere near as filling as if they were actually in a lasagne, so you would be likely to eat a whole lot more of them, and therefore eat a whole lot more calories.
like other's have mentioned it is when we do things with foods that we wouldnt normally do. Like substitute flour with things like cous cous/chick peas etc to make breads or cake, which are all things you could easily over indulge in.
The same for blending/cooking fruit, you could easily eat a whole heap of fruit very quickly in a smoothie say, but it would take you far longer to eat that amount of fruit in it's raw form and you are of course using up some energy chewing etc. Where as you skip that process if you cook/blend the fruit.
Making crisps with lasagne/potato. These need to be synned also, as if you were using the potato on a red day, because again you could easily overindulge on such items. I know potatoes are free. But there are potatoes in a packet of shop bought baked crisps and no one would eat those and not count them.
The free foods list is there to be used from unlimited of course, but it is still possible to consume over what you body physically needs if you don't stick to the plan. Hence why the 1/3 superfree rule was added, to ensure you dont have a plate filled with only carbs and proteins, which although still free are the higher in calorie free foods.
If you use couscous in the usual way, to make the basis for a meal, then it is free.
But if you use couscous to replace something which would have a syn value, like the flour in a cake, then it is not free.
If you ate a bag of Doritos, you would have to syn them. So therefore, if you make fake "doritos" out of sheets of lasagne (which would be free if you were making a meal from them), then you have to syn them.
Basically, if you are using an ingredient to try to avoid another one that you would have to syn, then that's what they call "food abuse" (although I hate that term and wish someone would think up something else for it).