TBh I don't know how the plan works I just know it does, I find it really easy and fits really well with eating with other people.
SW have spent millions of pounds and 40 years developing the plan so I'm not sure it can be explained.
All I can think to say is it limits calories without thinking about it, by eating low calorie dense foods in abundance and restricting the hig density calorie foods.
As an experiment I logged my intake on my fitness pal for a week or so and my calorie intake was approx 1500 a day consistently.
average loss varies person to person and how diligently they follow to plan, I lost 2.5 stone in 14 months but my consultant lost 3.5 stone in 7 months, so its up to you really.
Thing with SW is is that it becomes a way of eating and not a diet, its healthy and well balanced eating with the benefit of weight loss, sounds too good to be true but it is true, however its not a magic wand and you do have to work at it x