That's not actually so. You drain the whole system, use air to blow the water/coolant mix out of the heater core and hoses, etc., and if you're feeling really fastidious you wash out the system with denatured alcohol from the hardware store and then let it air dry (I've never bothered). Then you fill the system with Evans and for the next couple days to weeks after you run the car you burp off the steam from the residual water by loosening the radiator cap then retightening it. Once there stops being any steam pressure, you're done and that's it.
This just plain isn't true.