Same principal as putting a cup under water, turning it upside down, then lifting it almost to the top of the water. There's no air in the cup, so the water stays in it. Not something I'd ever want to do with a tank... imagine how hard water changes would be.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry