When the Katana is held correctly the left pinky finger is the most powerful finger in the grip. A stroke’s power originated in the legs and hips, traveled down the shoulder and into to the forearms with a extension of the elbows, and through the hands with a snap of the wrist. The fingers held the handle gingerly at the top and tightly with the pinkies allowing all the energy in a swordsman’s center to flow out his hands and down the blade to the point of impact. Properly executed, a stroke could severe skull.
If a Samurai was to have his pinky finger amputated his grip would be severely compromised and his technique diminished. Less ability with his sword would force him to rely more on the protection of the clan, and its lord, the daimyo. This was the origin of yubizume, cutting off the pinky as a sign of apology, still practiced by the Yakuza. A first offense would mean the top joint of the pinky, a second offense – the lower joint, and so on until the finger was gone and onto the next. Not many Yakuza lost more than one finger, those who so frequently fucked up were more likely lying in a bed of concrete than walking fingerless.