Thumb sticks out between first and second finger of the fist.The thumb fist concentrates the striking force on the tip of the thumb. It is only used against soft targets, such as the eyes or pressure points.
The one-knuckle fist concentrates the striking force on the middle finger knuckle; the index finger knuckle may also be used. It is often used in a twisting, "drilling" manner against pressure points.
The Isshinryu fist is an Okinawan variant devised by Tatsuo Shimabukuro, the founder of Isshinryu karate. He claimed that by locking the thumb atop the index finger, the thumb was protected and the wrist was stiffened.
The punch is launched from the side, keeping the fist vertical the entire time. The elbow is kept close to the side and the shoulder is relaxed and dropped. The punch is usually practiced targeted at the solar plexus. The punch strikes with the first two knuckles of the hand, keeping the elbow slightly bent, and is then snapped back, much like cracking a whip.
The bent elbow prevent its injury if the punch misses or practicing without striking an object. The bent elbow also maximizes the blow as it permits the punch to be driven into the body. The bent elbow limits this a close-range punch.
The fist is made by holding the hand open and then slowly curling the fingers from the most distal knuckle until tight fist is formed. Then the thumb is pressed down on the second knuckle of the index finger. Styles which practice a twisting punch frequently wrap their thumb down over the fingers, which is how how Shimabuku taught a twisting punch. Since Isshin-ryu punches straight ahead, the vertical thumb position allows for cleaner alignment of the wrist and arm bones, supports and protects the thumb, and allows it to be used for jabbing pressure points.