As coolant heats and expands, it has to go somewhere. If your radiator is full, the pressure release on the cap lets the coolant flow out to the overflow tank. Once the engine cools down, the coolant shrinks back down, and the cooling system is at a vacuum and thus will suck the coolant back in from the overflow tank. The overflow tank isn't pressurized, so you could substitute virtually any tank you can find a way to stuff the hose into.

TJC's truck has some air space above the fluid in the radiator that allows the coolant to expand without spilling out. If needed, air will be pushed out of the radiator cap and sucked back in as it cools. The only problem here, is that its harder to check the coolant level. With an overflow tank, if some leaks out, the system should be able to suck new stuff in after a warming/cooling cycle. You can monitor the level by checking how much is left in the overflow tank. This will only work for small leaks. Large leaks will just bleed out coolant and could suck in air, or not allow the system to pressurize and then suck when cooled down.

I just had to replace my water pump, and didn't get a chance to pick up new coolant before doing it. I recycled what I could back in, but was about 1/2-3/4 of a gallon low. I ran it about 2 weeks before I had a chance to stop and get some new stuff and had no problems.