At the beginning of a sumo match, the rikishi stand on their respective lines and when the signal is given they make their tachiai, which is their first meeting/approach. Sometimes they slam right into each other and get in a headlock (or do a mawashi grab) and sometimes they start slapping each other first. It's kind of loud, and it looks like it hurts.
No fist punching is allowed. This is not boxing or a fight club. But the slapping is acceptable, even expected. Their bronze skin gets a short beating before the two competitors latch onto each other or truly attack. It looks like it stings.
Here's a link to a youtube video of a slap fight.
In the video you can see the slapping is fierce enough to actually send one guy out of the ring. Plus you can see the decor of the sumo arena. Cool video--thanks Snowleg, whoever you are.