I've done Success Express before, usually only for a couple of days to get back on track after a bit of a blowout. The first time I lost weight with SW it fell of really quickly, and when i thought about it I think it was because I was following the principles of success express where i was having meat with mountains of vegetables (even though it was before the time of that plan).
Basically you're just filling up on the superfree foods and restricting your intake of the foods free on green or red days. It works very well when you're having dinner out somewhere, or cooking for the family, as you can have a shepherds pie, or lasagne with meat and pasta or potatoes, but only on 1/3 of your plate (it's still a very good sized portion!!), with the other 2/3 being superfree foods.