In professional Japanese sumo, before a bout starts - indeed before any of the stomping - you will see the two sumotori on the dohyo accepting a traditional ladleful of water (chikaramizu, lit. "strong water" or "power water") from a sumotori standing beside the dohyo, by the south-eastern and south-western1 corners respectively. One of these two wrestlers will be the winner of the previous bout, and the other will be waiting to participate in the next bout. The loser from the previous bout will have left the hall immediately after the bout, since losers are bad luck and not supposed to hang around the dohyo.
This way it is easy to see, that one of the sumotori in the last bout of the day won't have anyone to offer him the chikaramizu. The loser from the previous bout will have left the hall, and there are no more sumotori waiting to get in the fray.
This is where the kachinokori comes in.
The kachinokori is the winner of the third or fourth to last bout on either side. He stays behind after his bout, thus ensuring that the hand holding the ladle with chikaramizu will always belong to a winner, regardless of who wins the second to last bout.
- The banzuke, or ranking list, is divided into an East and a West side, with East traditionally being slightly more prestigious.
My sources are Sumoforum.net and countless hours of watching sumo.