The "Motobu Fist"
Choki Motobu was a famous Okinawan karate master in the early and middle 1900's. He was most well known for his dedication to the mastery of the simple makiwara, the Okinawan punching board. In his practice, he came up with a slight modification of the standard way of forming a simple fist, which is now known amongst karateka as the "Motobu Fist".
In this variation, the index finger is folded straight down, and not curled under with the other fingers. The advantages are twofold: the fist becomes smaller and tighter with the index finger moving back to allow the index knuckle to protrude, and the thumb is now backed up and supported by the folded index finger. If a regular fist is struck directly on the thumb, the thumb can be instantly pushed back into the hollow of the fist and dislocated or broken. With the Motobu fist, the thumb is well braced by the second and first bones of the index finger, and can endure much more force without damage.
The only real disadvantage of the Motobu fist is the extra effort in forming it correctly. With enough practice, it will become just as fast and automatic as a regular fist.
We highly recommend this variation of the simple fist, especially for students with larger fingers who have a hard time making a regular fist tight enough to hit with the front knuckles instead of the fingers.