Gripping

In the 12th Century Japan, the Shogunate system began. The head of a Shogunate was the Shogun. Under the Shogun were regional daimyo. Each daimyo had samurai who protected him and his realm. When a samurai displeased his daimyo, as punishment, he would have to endure yubizume, where the first joint of the little finger on the sword hand would be amputated. Although this was a painful punishment, cutting off any finger would have been just as painful, so why did the daimyo choose the little finger?

For major transgressions, the daimyo would simply have the samurai killed, but, for minor transgressions, since the daimyo needed the samurai for protection, he did not want to the punishment to render an unruly samurai incapable of fighting, so he chose to remove the little finger joint. Why did the daimyo choose the little finger?

The other fingers are needed for all types of work and fighting methods, while the little finger seems insignificant. This appears to be a good reason to choose the little finger for amputation, but why was cutting off the little finger more of a disgrace to the samurai than cutting off any other finger?

The reason is that, when holding a sword, the little finger is the finger with the strongest grip. Each finger subsequent finger has a weaker gripping strength than the one before it. Therefore, weakening the little finger's grip would substantially weaken the samurai's grip on his sword, making him a less capable samurai and thus, less a man.