Water Heaters - Water heater auto shutoff valve...

I am interested in installing an automatic water shutoff valve (ex. Flood Stop) for my water heater and had a question about it's function. I understand that if my tank starts leaking, this valve will cut off the cold water feed to the hot water tank. Will this stop the flow of the leak or will the contents of the water tank (40, 50 gallons etc.) still continue leaking until it is empty or reaches a level below the leak point? I guess 40 gallons of water leaking is better than if the tank was leaking endlessly since the cold water feed would continue to supply the water. Thank you for any info you can provide.
-Bill

The valve will shut off the flow of water into the tank. The contents of the tank can possibly come out if there is a leaking hot side valve or someone happens to open a valve.

Vaccum will hold most of the water in the tank.

Thank you very much for the reply!! I have also considered getting a "Watercop" type shutoff which would cover possible leaking in several areas at once but think I will go with the auto shutoff for the hot water heater and one for the washing machine. Any thoughts as to whether the "Watercop" would be necessary over just covering the most common/likely to cause the most damage areas?