Sasaki Kojirō was a long-time rival of Musashi Miyamoto, and is considered the most challenging opponent Musashi ever faced.
There are a number of accounts of the duel, varying in most details except the essentials, such as Kojirō's defeat.
The age of Kojirō is especially uncertain - the Nitenki says that during his childhood, he
...received the instruction of Toda Seigen, a master of the school of the short sword, and having been the partner of his master, he excelled him in the wielding of the long sword. After having defeated his master's younger brother he left him to travel in various provinces. There he founded his own school, which was called Ganryu.
The Nitenki's account initially seems trustworthy, until it goes on to give the age of Kojirō at the time of the duel as 18 years old; it is known that two years earlier he had been a head weapons master for a fief - but then that would imply he had reached such a position at the age of 16, which is extremely improbable. A further complication is that Toda Seigen died in the 1590s. This unreliability of the sources means Kojirō's age could have varied anywhere from his 20s from to as late as his 50s. Even worse, a number of scholars contend that identifying Seigen as Kojirō's teacher is a mistake, and that he was actually trained by a student of Seigen's, Kanemaki Jisai.
Apparently, the young (at the time, around 29 years old) Musashi heard of Kojirō's fame and asked Lord Hosokawa Tadaoki (through the intermediary of Nagaoka Sado Okinaga, a principal vassal of Hosokawa) to arrange a duel. Hosokawa assented, and set the time and place as 13 April 1612, on the comparatively remote island of Ganryujima of Funashima (the strait between Honshū and Kyūshū). The match was probably set in such a remote place because by this time Kojirō had acquired many students and disciples, and had Kojirō lost, they would probably have attempted to kill Musashi.
According to the legend, Musashi arrived more than three hours late, and goaded Kojirō by taunting him. When Kojirō attacked, his blow came as close as to sever Musashi's chonmage. He came close to victory several times until, supposedly blinded by the sunset behind Musashi, Musashi struck him on the skull with his oversized bokken (wooden sword), which was over 90 centimeters long. Musashi supposedly fashioned the long bokken, a type called a suburitō due to its above-average length, by shaving down the spare oar of the boat in which he arrived at the duel with his wakizashi (the wood was very hard). Musashi had been late for the duel on purpose in order to psychologically unnerve his opponent (a tactic used by him on previous occasions, such as during his series of duels with the Yoshioka swordsmen).
Another version of the legend recounts that when Musashi finally arrived, Kojirō shouted insults at him, but Musashi just smiled. Angered even further, Kojirō leapt into combat, blinded by rage. Kojiro attempted his famous "swallow's blade" or "swallow cut," but Musashi's oversized bokken hit Kojiro first, causing him to fall down; before Kojiro could finish his swallow cut, Musashi smashed Kojiro's left rib, puncturing his lungs and killing him. Musashi then hastily retreated to his boat and sailed away. This was Musashi's last fatal duel.
Among other things, this conventional account (drawn from the Nitenki, Kensetsu, and Yoshida Seiken's account), has some problems. Would Musashi only prepare his bokuto while going to the duel site? Could he even have prepared it in time, working the hard wood with his wakizashi? Would that work not have tired him as well? Further, why was the island then renamed after Kojirō, and not Musashi? Other texts completely omit the "late arrival" portion of the story, or change the sequence of actions altogether. Harada Mukashi and a few other scholars believe that Kojiro was actually assassinated by Musashi and his students - the Sasaki clan apparently was a political obstacle to Lord Hosokawa, and defeating Kojirō would be a political setback to his religious and political foes.
The debate still rages today as to whether or not Musashi cheated in order to win that fateful duel or merely used the environment to his advantage. Another theory is that Musashi timed the hour of his arrival to match the turning of the tide. He expected to be pursued by Sasaki's supporters in the event of a victory. The tide carried him to the island then it turned by the time the fight ended. Musashi immediately jumped back in his boat and his flight was thus helped by the tide.
His favorite technique was both respected and feared throughout feudal Japan. It was called the "Turning Swallow Cut" or "Tsubame Gaeshi" (燕返し lit. "Swallow Reversal / Return"), and was so named because it mimicked the motion of a swallow's tail during flight as observed at Kintaibashi Bridge in Iwakuni. This cut was reputedly so quick and precise that it could strike down a bird in mid-flight. There are no direct descriptions of the technique, but it was compared to two other techniques current at the time: the Itto-ryu's Kinshi Cho Ohken and the Ganryū Kosetsu To; respectively the two involved fierce and swift cuts downward and then immediately upwards. Hence, the "Turning Swallow Cut" has been reconstructed as a technique involving striking downward from above and then instantly striking again in an upward motion from below. The strike's second phase could be from below toward the rear and then upward at an angle, like an eagle climbing again after swooping down on its prey.