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If a yakuza member displeases or severely disappoints his boss, the punishment is often yubizume, the amputation of the last joint of the little finger. A second offense will require the severing of the second joint of that finger, and additional offenses might require moving on to the next finger. A man knows that he must commit yubizume when his immediate superior gives him a knife and a string to staunch the bleeding. Words are not necessary. The origin of this practice dates back to the days of the samurai. Removing part of the smallest finger weakens the hand for holding the sword. When a katana (the samurai long sword) is gripped properly, the pinkie is the strongest finger. The ring finger is the second strongest, middle finger third strongest, and the index finger does almost nothing. With a damaged hand, the swordsman became more dependent on his master for protection. Today this ritual maiming is entirely symbolic, but it serves to make a point with delinquent kobun, and it shows that the yakuza, like their Mafia counterparts, abide by the old saying: “Keep your friends close and your enemies closer.”
Taken from here: