It's called Mawashi, not diapers.

Sumo is a competitive contact sport where a wrestler (rikishi) attempts to force another wrestler out of a circular ring (dohyō) or to touch the ground with anything other than the soles of the feet. The sport originated in Japan, the only country where it is practiced professionally. The Japanese consider sumo a gendai budō(a modern Japanese martial art), though the sport has a history spanning many centuries. The sumo tradition is very ancient, and even today the sport includes many ritual elements, such as the use of salt for purification, from the days sumo was used in the Shinto religion. Life as a rikishi is highly regimented, with rules laid down by the Sumo Association. Professional sumo wrestlers are required to live in communal "sumo training stables" known in Japanese as heya where all aspects of their daily lives—from meals to their manner of dress—are dictated by strict tradition. Mawashi are worn by rikishi during their trainings or competitions.

For top, sekitori, a sumo wrestler who is ranked in one of the top two professional divisions: makuuchi and juryo.Currently there are 70 rikishi in these divisions...., ranked professional rikishi, Mawashi is made of silk.It is approximately in length when unwrapped, about two feet wide and weighs between eight and eleven pounds. It is wrapped several times around the rikishi and fastened in the back by a large knot. A series of matching colour, stiffened silk fronds, called sagari are inserted into the front of the mawashi. If these fall out during competition the gyoji, a Gyoji is a referee in professional sumo wrestling in Japan.

Sometimes, a rikishi may wear his mawashi in such a way as to give him some advantage over his opponent. He may wear it loosely to make it more difficult to be thrown or he may wrap it tightly and splash a little water on it to help prevent his opponent from getting a good grip on it. His choice will depend on the type of techniques he prefers to employ in his bouts. Thus a wrestler preferring belt sumo will usually wear it more loosely, while those preferring pushing techniques will tend to wear the mawashi more tightly.Many rikishi are superstitious and they will change the color of their mawashi to change their luck. Sometimes a poor performance will cause them to change colors for the next tournament, or even during a tournament, in an attempt to change their luck for the better.The rikishi only wear the silk mawashi during competitive bouts either during ranking tournaments or touring displays. During training a heavy cotton mawashi is worn.

For senior rikishi in the top two divisions (the so-called sekitori) this belt is coloured white, and it is worn with one end distinctively looped at the front. Sagari are not worn during training.Rikishi ranked in the lower divisions wear a black cotton mawashi both for training in and in competition. In competition cotton sagari are inserted into the belt, but these are not stiffened.Amateur sumo wrestlers are expected to wear a white cotton mawashi, without the looping accorded to the senior professional's training garb.If a wrestler's mawashi comes off during a tournament bout, he is automatically disqualified. This is of course extremely rare, but did occur in May 2000 when sandanme wrestler Asanokiri was embarrassed during a match with Chiyohakuho.