Best known as the garb of sumo, the mawashi is a thickly padded variation on the fundoshi -- basically a 4-layer fundoshi -- that is not only to clothe and protect the wrestlers, the mawashi itself is a big part of sumo wrestling! Some variation of the mawashi is also often worn during Japanese festivals, either in place of fundoshi or as one of dozens of different hybrids bewteen the two types of loincloth.
In sumo, a mawashi (Japanese: 廻し) is the belt that the rikishi (or sumo wrestler) wears during training or in competition. For top- (sekitori-) ranked professional rikishi, it is made of silk and comes in a variety of colors. It is approximately 30 feet (9.1 m) 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.
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. 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 extremely rare, but did occur in May 2000 when sandanme wrestler Asanokiri was embarrassed during a match with Chiyohakuho. It should be noted, however, that for most of Sumo's history, whether or not a wrestler's mawashi came off during a bout was considered irrelevant, and the policy of disqualification only came in to place when Japan began adopting European attitudes towards nudity.
Putting on a mawashi seems like a two-person job! The mawashi 30-feet long after all! I feel like this diagram is woefully inadequate, but it does help piece together the process. After passing the cloth through your legs, it is wrapped around the waist at least 4 times. After the first wrap, the excess "apron" in front is dropped -- the subsequent wraps go over the top of it. It can then be folded into the belt, if desired, to create the small triangle shown as B-5. This also exposes the pouch of the fundoshi. In the back, the excess end of the fundoshi belt is passed under the thong, then up the back by pulling it under the layers of belt. Finally, this is cinched through the portion that was passed through the thong, to create the knot at the back. It seems a matter of importance to keep everything flat, in fact I've seen a photo of a wearer carefully creasing his mawashi's fabric on a wooden floor by burnishing it with an empty beer bottle to make the creases extra-crisp.