Sodium does not suck water from the cell this is incorrect. The cell ie if we are talking muscle cells which we should be in BB will maintain homeostasis if it has adequate sodium and potassium. The body maintains a water hierarchy and will move water according to that hierarchy because that is essential to maintain life...the bodies number one priority.
The body functions in feedback loops, including water homeostasis, much like the thermostat and the air conditioner in your house, car, etc.
If you set the AC to 70 (its set point), the temp must reach some threshold (say 72) before it kicks in. It will cool until it reaches some value below its set point, say 68. There is an overshoot in the system. There must be some off-set or error to turn the AC on when the temperature reaches 72, and as a result it will cool to provide somewhat of a temperature buffer (for example 68 or 2 degree below the set point of 70).
Your body works much in the same way: imagine how your inner thermostat would react if you turned a heater on near the thermometer, but on the other side of the room from the AC. The heater would blast the thermometer (sensor) and the AC would be going full blast to cool the room. If you cut the heater off, the "smart" thermostat would not just cool the room to 68 - it would cool it to some level below that, because it remembers that there's a "threat" of heat AND the thermometer still hot, even though most of the room is at or below 70. (It senses a hotter room than is actually the case!) Water homeostasis is a bit more complicated, but you can use this principle to trick the body into drying out - losing water to a level of dehydration below its normal set point.
When discussing diuretics and water retention it only seems logical to dispel another pre-contest myth: Salt is a bodybuilders’ friend, not enemy, pre-contest. Many bodybuilders eliminate sodium like an ex-spouse at a honeymoon assuming the result will be the coveted ‘dry look’ on contest day. When salt intake is reduced, a series of ‘dry look’ nemesis arise. Salt contains sodium, and to a less degree potassium in the form of potassium iodide. When salt/sodium is reduced or eliminated from the diet the result is increased Aldosterone release. This makes the body excrete more potassium and hold more sodium/water. The resulting water retention gives the athlete a puffy wet look. This is due to electrolyte imbalances.
Reduced slat intake also negatively affects the all important sodium-potassium pump. This is the mechanism the body used to shuttle many nutrients into cells like those that all muscle fibers are composed of. (Gee, ya think?) This would therefore inhibit creatine and some amino acid structures from adequately transporting, as well as inhibit glycogen synthesis. If the salt content is reduced in muscles so is the water content. This means catabolism, flat muscles come show time, and a lack of vascularity. (It would also inhibit erectile function, but that is another issue altogether.)
The key to subcutaneous water control depends upon control of the hormone aldosterone. Obviously estrogen control is part of this hormone cascade action/reaction. But, our main focus is sodium and water control, so aldosterone is the key.
While pre-contest dieting an athlete should have a relatively high sodium intake. This creates an environment in which the body does not have to release Aldosterone. This causes sodium to stay in the muscle tissue and the subsequent attraction of water stores there. Also, the all important maintaining of the sodium-potassium pump is accommodated as well. (During diet phases, this also reduces catabolism). At the same time water intake should be relatively high as well. This helps your body excrete any extra sodium, which of course it will, because Aldosterone secretion in the body has been controlled by elevated salt intake/water intake. The body will continue to dump all excess water and sodium as long as this is followed.
T