Despite what fad diets promise, it's not possible to safely lose 20 lbs in 10 days nor would the weight loss be sustainable. However, athletes are known to lose approximately that much weight in about that span of time by temporarily dehydrating themselves to an often dangerous extent.
Medical professionals are in agreement that a safe rate of weight loss is between 1 and 2 lbs a week, although that rate may be temporarily higher, between 3 and 5 lbs, for people who are especially overweight when they begin. This kind of weight loss occurs mostly as the loss of adipose tissue (fat) over time and is accomplished by moderate, consistent exercise and restricting calories. Food consumption is reduced by about 500 calories per day to achieve this weight loss.
Professional fighters, on the other hand, frequently cut 20 lbs or more in the span of one or two weeks before a competition, so they can "make weight" at a given weight class. This is accomplished via dehydration, which they achieve by carefully limiting their intake of carbohydrates, electrolytes and water. They also use saunas, vigorous exercise and other sweat-producing activities to get the water out. Once they have successfully weighed in, they begin rehydrating immediately and regain most of the lost weight by the next day.