In order to sucessfully keep off the weight, Cambridge advise you to use the step system. If you are unfamiliar with this, basically there are 6 steps: SS, 810, 1000, 1200, 1500 and maintenance. Sole source is the lowest step, and means you completely replace conventional eating with the Cambridge packs. As you move up the plans you gradually start to introduce food again.
If you spend at least 2 weeks on each step of the programme, and keep moving up there is no reason to pile all the weight back on again, providing you don't fall back into your old eating habits. It is about adoping a new healthy approach.