Most bouts are won at the start, the moment of tachi-ai. - Yuzuru Hiraga, John Swain, John Crossley
The meaning of tachiai is to surge the fighting spirits facing and staring at each other, seeking a synchronized timing to start the bout. The rikishi may start any time if they are in mutual (implicit) agreement to do so.
In the old days, there was no time limit on a tachiai. As would be expected, there were culprits who wouldn't go, out of tactics to tantalize the opponent, and tachiai would take ten minutes or longer. Thus, the last bout might go well into the night. Now, there are time limits on each tachiai, which was introduced with the advent of sumo broadcasting in the radio days. I believe the limit was three minutes. I also think it's variable so that the 6:00 pm limit for the television broadcast can be met.
Starting in 1950, the time limits were: Makuuchi 4 minutes, Juryo 3, and Makushita two minutes.
The time to start the bout comes when the gyoji shows the other side of the gunbai (war paddle, or fan), calling "gunbai wo kaesu" (flip the gunbai). The time, though, is kept by one of the shimpan (referees sitting by the dohyo). The first and more visible sign that time ha come is that the yobidashi stands up and informs the rikishi. Yobidashi are the fellows that call out the rikishi's names, clap the sticks (hyoushi-gi) together, sweep the dohyo, and do other all sorts of tasks. They're the ones that build the dohyo before a tourney, too.
Even when time's up, it's still up to the rikishi as to exactly when they go. Now they can no longer leave the dohyo to take salt or water. If one carelessly steps out, he automatically loses the bout. There actually were a couple of these incidents in the past.
The beginning of the bout is communicated partly through the hands touching the dohyo; both rikishi are supposed to touch both hands before starting.
A false start (matta) happens when one rikishi starts before the other. There is a 100000 yen fine for this: one or both rikishi may be penalized, depending on later judgment. Usually, the one who makes a premature start gets fined for not synchronizing with the other (kokyuu wo awaseru).
Once the rikishi start to grapple, the outcome can be called by one of the seventy Kimarite ("The 48 Hands") and the techniques used described by oshi, yori, nage and so on.
During the action of a match, the referee (gyoji) tells the wrestlers they are still in play by yelling "Nokotta!" When rikishi go into a clinch or are otherwise at a standoff, the gyoji will encourage them to move it along by calling out "Yoi, Hakkeyoi!"
Sometimes two rikishi touch the dohyo at the same time, yet the gyoji still has to indicate a victor. If the judges want to talk it over, they will call "mono-ii" and climb up onto the dohyo to discuss it. The outcome of the deliberation can be to uphold the referee, reverse that decision, or call for the two rikishi to replay the match.
Striving for the middle path, a simple bare majority of wins is the goal of any rikishi. Too many wins will result in a promotion that might put one in tough company next basho. On the last day of a tournament (senshuraku) rikishi with even records (7-7, for example) have a curiously high chance of coming away with an eighth win. This is certainly some kind of coincidence, a simple statistical anomaly.
When two or more rikishi finish the basho in a tie for the lead, they participate in a kettei-sen (playoff). This is equivalent to "King of the Mountain." No matter how many rikishi are tied, the only way to break the tie is for one rikishi to beat all others in succession. Thus, if only two rikishi are involved, the play a single bout and the winner gets the yusho.
If three rikishi are involved, they draw from three papers which have one of the following marks: Higashi (East), Nishi (West) or a circle. The East and West rikishi then face off and the winner of that match then faces the rikishi who sat out the first match. This continues until one rikishi compiles two consecutive victories. If four rikishi are involved (I think I read that this has happened a few times), then they draw lots and compete until one rikishi compiles three consecutive victories.
In the Makunouchi division, a two or three man playoff might happen twice a year. In the lower ranks where there are hundreds of competitors and only seven bouts, playoffs involving as many as half a dozen rikishi are not uncommon.
If a rikishi is injured during the course of a Hon-basho and is unable to continue, he forfeits the rest of his bouts, which are figured as losses in his official record. When the next banzuke comes out, the injured man may elect to sit out that basho without being penalized as if he had fought and gone winless. Only one such basho can be skipped. Injuries sustained on Jungyo or off the dohyo do not qualify a rikishi for the Public Injury Rule.
When a Maegashira ranked rikishi defeats a Yokozuna, this goes into the record books and payroll ledger as a Kin-boshi, a "gold star" victory. Currently, Akinoshima holds the record number of these, with 15. Popular Sekiwake from the seventies, Hawaiian Takamiyama (now Azumazeki oyakata) racked up 12 in his career.
Any sekitori in the Makunouchi division under the Ozeki rank who finishes with a kachi-koshi is further eligible for any of three special prizes. These can be shared between two or more rikishi, or not awarded at all. They are as follows:
Shukun-sho, the Outstanding Performance prize: usually awarded when a man defeats a Yokozuna or the eventual yusho winner.
Kanto-sho, the Fighting Spirit prize: when a rikishi demonstrates superior vigor in a Hon-basho, the Kyokai may award this prize.
Gino-sho, the Technique prize: given for superior expertise displayed during the basho. Mainoumi, a "department store of technique," was jokingly said to be capable of winning the Gino-sho every time he got kachi-koshi. He currently has five of these.