The bow twirling? Back in the Edo era (1600~1850's) the champion of the day used to be presented with a bow as a victory prize. To show their satisfaction at received a nice prize, they did this dance of appreciation. Nowadays, it's one of the lower ranked wrestlers who does the dance. He enters the ring from the side of the last victor of the day.

Also, if he drops the bow, he is not allowed to pick it up by his hands, he has to use his feet. I've only ever seen that happen once.

If you're interested in sumo, check out the official page:
http://www.sumo.or.jp/en/index

it's called the yumitori-shiki