If you have the good fortune to attend a day of professional sumo in Japan you will be sure to notice the stomping and glaring that goes on on top of the dohyo, before the actual sumo begins.
Shikiri can mean something along the lines of "drawing the line" or "toeing off the mark". And that is what the wrestlers, or sumotori, do: they crouch down and put their fists on the ground by the shikirisen (starting lines), get up and walk back to their corner, perform kiyome-jiro - the throwing of the salt (originally to ward off evil spirits) - go back to crouch down again, and so on. They will lean very far forward, supporting their weight on their knuckles, making it look like they could do a handstand with no effort whatsoever. It can be a terrific display of strength, meant to help the wrestlers gain the mental upper hand and boost their spirit.
During the shikiri the gyoji (referee) will be standing with his side turned to the center of the ring, facing east (that'll be to the left, seen on tv, since the main cameras are placed north of the dohyo). But at some point he will turn and face the center, and that's the sign that time is almost up. The wrestlers will crouch down one final time, and seconds later we will have tachiai - the start of the bout.
My sources are Sumoforum.net and countless hours of watching sumo.