If you ever watch a live Japanese sumo match you will see a lot of interesting stuff that, more often than not, gets edited out before the sumo is shown on tv - at least outside of Japan. You'll see the stomping (shiko), and the baleful staring (niramiai), the throwing of salt (kiyomejiro) - and you'll see, briefly, the ritual mouthful of water, or chikaramizu.
The sip of water is offered to the sumotori (the wrestler) by a fellow sumotori - who will be either the winner of the previous bout, particpant in an up coming bout, or, if it's the last bout for the day it could be the kachinokori. Chikaramizu can be translated as "power water"; it is, of course, regular water, but it is, as almost everything else regarding sumo, heavily laden with tradition. The mouthful of water is not swallowed; the sumotori will rinse his mouth with it, and spit it out while politely covering his mouth with a small paper towel, (chikaragami).
My source for this one is, besides hours upon hours of watching sumo: Sumoforum's wonderful Glossary.