Dieting: Starving Yourselff.?

okay, they say if you starve yourself, your body goes into a certain mode where you savior your body fat slowly. But hw is it when you get gastric byp you lose all that weight? Because your body gets full bc there is little space. So why doesnt your body go into remission then bc its such a small portion then your used to? So if i drink alot of water and eat little bit and just not eat, it will be the same equivalance?