Before each bout, a sumo wrestler needs to have his hair specially styled. The topknot is the traditional hairstyle for all sumo wrestlers. It can be plain, or in the case of a rikishi in a more advanced rank, the topknot can be shaped like a ginkgo leaf (a fan shape.)
It takes over an hour to do up a proper chonmage (topknot.) The hair needs to be fairly long. It is then oiled generously and This is accomplished by a professional sumo hairstylist, or tokoyama. The tokoyama trains for years before becoming an official hairstylist to the athletes. The Japan Sumo Association ranks the tokoyama according to experience and skill, and only the best are allowed to style the hair of the top-ranked (sekitori) wrestlers.
Here is a picture of the ginkgo leaf style worn by the sekitori. (Champion level wrestlers.)
A site (that looks Dutch or German) I found about the chonmage said the type of oil the tokoyama use is called Bintsuke, and it is chamomile oil. The wrestlers must smell dreamy.