As with all diets no matter how complex they are they all come down to one thing... they put you in a calorie deficit.

So whats the difference with the 6 week diets?

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 Short term "Diets" for 9 days up to 4/6 weeks massively reduce your calorie consumption, they also reduce your nutrient intake also. So what actually happens is you drastically drop weight through massively restricting your foods and nutrients, practically starving yourself and you find you crave more. This in turn eats into a little bit of muscle so you actually stop dropping body fat and start eating in to Lean Muscle. But never the less, the scales don't know what is body fat or lean muscle - it just shows you dropping weight.

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 So by the end of the diet you feel horrendous due to a restriction in calories, food and nutrients. You still have a large portion of body fat but you also feel weak. 
Then after the "Diet" you over consume in calories and start putting weight back on at a very quick rate.

Within 2-3 weeks your back at the weight you started at before your diet.

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 The problem with the word "Diet" is it psychologically means it has a start date and end date. If you make sensible lifestyle changes to your nutrition you can lose weight but still enjoy your food. 
This is why at Fitness Plus our Nutrition Programme consultations are becoming increasingly popular. Because we actually structure the nutrition around the client so it enables them to still enjoy social activities due to us educating them about food, food choices, food qualities and quantities. Meaning they lose weight much more sensibly than starving themselves for a period.