When I lived in Tokyo with my older brother, he got into sumo, and then I did, too, once I got over the big-men-in-diapers ick factor (it helped to learn the proper name for the diaper: mawashi). We'd watch "Sumo Digest" whenever there was a basho (tournament) going on (they go for 14 days, six times a year). We took turns waiting in line for days to get tickets to one day of the basho advertised in the poster above. There were a lot of minor yakuza waiting with us, and we learned from our linemates and students that many corporations snap up blocks of the best tickets.
Sumo is an ancient sport that's connected to the Shinto religion as well as Japanese military and imperial history, and it's a test of strength and skill as well as a ritual and an exercise in psyching out your opponent. Check out these guys, preparing to fight:
There's a good five minutes of ritualized stomping and salt-throwing, and rikshi (wrestlers) crouch down at the face-off point many times before they actually go at it. They both have to have their knuckles on the ground before the match can begin, so crouching but keeping your hands off the ground is one way to stall the beginning of a match.
Things are pretty tense by the time a match begins, and it can go very fast from there, because the rules are very simple: Force your opponent outside the ring, or make him touch the ground with more than the bottoms of his feet, and you win. Approved fighting techniques include grabbing the other guy's mawashi and using it for leverage, shoving, slapping, hooking the other guy's leg with your leg, and so on.
This clip is an excellent example of a classic bout:
These nobori outside the kokugikan (tournament hall) announce the honbasho (Grand Sumo Tournament):
Finally, it's definitely a family affair; this is one of many kids we saw the day we went: