Everybody knows that the samurai were a respected class in Japan. They were so respected that the law allowed them to kill somebody if they showed no respect to them. The samurai sword needed to be equally as good as them. Because of that, a Japanese sword had to be extremely sharp and durable.
To demonstrate the quality of the sword, they needed somebody special to make it, because a novice could make wrong the cut. By choosing the best swordsmen, they could avoid that situation and it was easier to know which sword was the best. The results were more credible this way.
Normally, the sword was tested on things that would resemble the human body; the material for imitating this “body” was bamboo and tatami mats.
This test is called “Tameshigiri” and literally means cutting test. The different types of cuts were named after the area where the body was cut, as were tabigata (cut at the ankle) or ni no do (cut halfway of the body).
Considerable controversy exists about whether some of these cutting tests were done on human bodies, as some books say that only a few swordsmen had access to them, while others say it was a very common practice.
What seems to be true is that until 1700, this practice was accepted and many samurai paid for bodies of criminals in order to test their sword and practice their cutting techniques.
The demand for bodies was so high, that in some places it was forbidden to use the bodies of the samurai, monks and women for the sword testing.
Once the samurai became part of a more privileged and less warlike class, the lower samurai class performed this technique.
A samurai was the only one who could perform the katana technique on bodies. A samurai would never give his sword to someone else to perform a cut test on a body, as this would be humiliating and dishonorable for him.
There is a curious story in which a murderer asks the samurai if he could use the sword to kill him. The samurai answered that he would cut him in at the height of his stomach with the sword because he needed to test it. The murderer answer that it was a shame he didn’t know this before, otherwise he would have swallowed a couple of large stones in order to make his sword blunt.