sentri, it's called water loading. You get your body accustomed to a certain amount of water, and get it used to expelling that water. When you cut that amount entirely, your body flushes all the water out and doesn't retain as much because it's become habitual. At least I think that's how it works, tricking your body into creating a natural diuretic effect. It works the same way as sodium loading, I guess.
Peace.