The first and most famous version of this diet was The Cabbage Soup Diet where you have the soup three times a day. The idea is that the soup will be filling and you will eat fewer calories. Just as in the Juice Diets, you will lose weight temporarily from muscle and fat. You will also lose water and salt which weighs 1 kilogram or about 2 pounds per liter or quart. Water loss accounts for some of the large short term weight losses claimed in the first week of these plans.
Learn more about diet myths at https://www.uclahealth.org/clinicalnutrition/nutrition-fact-or-fiction