That's an interesting theory and you may be right although differently worded from what I understand. Unlike a "flush", if you drink only as thirst demands during a fast, you won't be flushing anything very far as that fluid will be absorbed and used by the body. Excessive drinking will result in some water not being absorbed but it then becomes an extra bit of waste for the body to deal with instead of resting and recuperation.
I haven't researched dry fasting yet and until I get to it, I can only look upon it suspiciously.