As the technique's name suggest, Yontoryu is swordsmanship that uses four swords. Kaku achieves this by combining his two shirasaya katana with his Rokushiki technique "Rankyaku", which lets him create slicing air blades via kicking, thus appearing to have four blades in total. Kaku's large variation in the Rankyaku technique makes the Yontoryu a truly powerful and versatile combat style. The most prominent example of combining his two swords with his Rankyaku is the "Rankyaku: Ran", that uses the two blades to stay on the ceiling while raining a flurry of air blades down at his opponent. However, Kaku can only maintain Yontoryu if he is in control of the fight; if forced on the defensive, as seen at the beginning of his fight with Zoro, he can only defend with two swords. However, when transformed to his half-giraffe form, his reach is long enough to make it largely impossible to force him on the defensive.
Upon consuming the Ushi Ushi no Mi, Model: Giraffe, Kaku further attained versatility with his Yontoryu style, by combining the increase in size and strength that he gains from transforming into a giraffe-human hybrid. His long neck also serves as a whip of sort, which he functions somewhat like a blade.
In the episode when Vivi leaves the Straw Hats, the crewmembers were very sad; when Zoro suggests they should have kept Vivi by force, each Straw Hat jeers at him. Luffy's jeer was 'Santoryu', but then changed it to 'Yontoryu' when Usopp mentioned that 'Santoryu' wasn't an insult.
↑One Piece Manga and Anime — Vol.
24 Chapter 217 (p. 16) and Episode 130,
Luffy fails to insult Zoro by nicknaming him "Santoryu", but when Ussop corrects him that it isn't an insult he adds one by nicknaming him "Yontoryu".