Roronoa Zoro's Nine Sword Style in Japanese is "Kyutoryu," and it is certainly an extraordinary technique that has been regarded as the epitome of his swordsmanship in the One Piece series. This dark power ability was an extended version of Zoro's Three Sword Style and, much like the name suggests, involved the use of nine swords at one time. But the nature of the technique is even more complex and mystical than just a simple increase in the number of physical swords.
Nine Sword Style does not pertain to the physical wielding of nine swords, more of the incarnation that comes forth out of Zoro's never-say-die spirit and his will. By mere power of will, Zoro creates an illusion of two pairs of extra arms and two extra heads, also yielding a number of swords which sum up to appear as nine. This spectral image is often represented as a demonic or divine figure, somewhat resembling the Buddhist mythical Asura, and because of that, the technique is also called "Asura."
Many One Piece fans have debated the way that Nine Sword Style works. Generally speaking, it is believed that these are not extra arms and heads at all but rather the illusions that appear in the minds of other people as a product of Roronoa Zoro killing intent and fighting spirit. This interpretation fits in very nicely with the series' view that willpower is an important factor in determining a character's actual strength or ability.
For combat, the Nine Sword Style enhances Zoro's ability to attack opponents. Because he can create the illusion of using nine swords, Zoro can literally shower his opponents with strikes from every direction, increasing his capability to handle multiple types of attacks at once. The result would have sufficient strength or speed to deal with a powerful enemy counterattacking.
One of the most remarkable demonstrations of Nine Sword Style was during the Wano Country arc, where Zoro fought against Kaido, one of the Four Emperors. This battle, shown in episode 1027 of the anime, is a brilliant example of the power of the technique against one of the strongest characters in the One Piece world. There is no need to say that using Nine Sword Style in such an extremely critical fight like this one puts the seal on it being one of Zoro's ultimate techniques.
It is important to note that, as powerful as the Nine Sword Style is, it is actually not Zoro's standard fighting style at all. Most of the time, he either resorts to his One Sword Style or Three Sword Style techniques. The Nine Sword Style is almost used only in extremely desperate situations or against very strong enemies. This use adds to the mystique and effect of the technique when it finally does appear.
The animation of the Nine Sword Style in the anime and manga of One Piece is very vivid. The image called to mind is that of Zoro surrounded by translucent arms and heads, all brandishing swords, to create an aura of power and being definitely intimidating; this has made this technique really iconic to the series, often featured in promotional material and fan art.
Of course, even the mighty Nine Sword Style doesn't come without controversy in fandom. Some would say Zoro is "holding back" whenever he fails to use this technique in every fight. But then again, such a view probably forgets about the probable physical and mental stress of sustaining such a powerful illusion and the strategic value of saving some of the strongest techniques for truly tough battles.
Zoro's creation of the Nine Sword Style brings out his growth throughout the series as a swordsman. This is symbolic of the transcendence of human limit as often seen in One Piece. Development of this apparently superhuman power without the consumption of a Devil Fruit makes it even more unequivocally clear how talented he is and how sheer his will is.