I think it's because (unless they changed the Fast Forward rules in later plans) the FF has you drinking the juice and eating meat and green vegetables. You lose a bunch of water weight while you are getting rid of your sugar and salt cravings. The Take Off plan, which (in the olden days, anyway) you do every two weeks afterwards, or as needed, depending upon your weight loss, has a small amount of protein, some vegetables, the juice and your two lites.
Back when I went to the centers, there was also a plateau breaker menu; pretty straightforward. They encouraged you to eat chicken or fish for protein, lots of green vegetables, berries and citrus for fruit servings, and light bread and crackers for your starch servings for five days. You used the same portions as your regular plan.
I think that whatever amount of weight you lose on Fast Forward or Take Off, it is still a good thing to do because it helps you get rid of your cravings and make you ready for the regular diet.
I always get a little headachy the first day of Fast Forward because I am getting the sugar out of my system and my body rebels a little.