How does breastfeeding help with weight loss?

Milk is high caloric. "dieting" is not needed, as long as the breastfeeding woman eats only as many calories as needed to maintain her normal (pre-pregnant) weight. For example, if she was 120 pounds before pregnancy and is now 145 pounds, she should eat just enough for a 120 lb woman. Her 25 lb excess will be gradually used up in the production of breastmilk (25 ounces of breastmilk contains about 500 calories).
It doesn't always. If you don't follow dr. Kwok's recommendations for limiting caloric intake, you won't lose weight.