How can it be that replacing a drain valve in a full tank leaks not water?

I still feel badly that I didn't replace the plastic drain valve with the
brass ball valve because I was afraid the plastic was not removable (that's
what Sears salespeople said anyway, instilling FUD in my mind).

I was subsequently reading how to replace the plastic drain valve after the
fact and they all seem to say it won't leak if I do so with a full tank.

Huh?

How can removing the drain valve at the bottom of a full water heater not
leak 50 gallons of water?

I already saw, first hand, what happens when that drain valve
http://www.flickr.com/photos/donnaohl/2274079114/
snaps off
http://www.flickr.com/photos/donnaohl/2273262545/
spewing 40 gallons of hot water in my garage
http://www.flickr.com/photos/donnaohl/2273262535/

Can someone clarify this statement from, say:
http://216.250.104.54/default.cfm?PageId=41781 (see excerpt below).

Donna


Maintenance Tip #21 Drain valves
Drain valves usually come when you buy them from a manufacturer. They used
to be brass. You should install a 3/4 ball valve on your water heater.
The plastic ones are prone to leaking. To install a brass ball valve, turn
the gas or electricity off and open a hot tap inside the house. If you
have a cone-shaped valve, unscrew it counterclockwise six turns or so and
pull it out at the same time. Now turn it clockwise while continuing to
pull and it will come out. Wrap Teflon on the nipple that is exposed on
the water heater. Attach the ball valve now. If you have a plastic drain
valve that looks like a hose bib, unscrew it by turning the entire valve
itself.
A little water may come out while you're installing a new valve, but not
much at all. Wear gloves to avoid getting scalded.