A punch begins with the fist on the hip, with palm facing upwards and the elbow facing back. The fist must travel in a straight line to the target on the centerline of the opponent's body. The forearm and elbow scrape the side of the body as they pass. The elbow remains down and relaxed until the moment of impact when the fist rapidly turns over to face palm down.
Even after years of study, an adult black belt can find that his punch has grown sloppy, and these basics have not been adhered to.
Notice in the picture how the elbow has swung out to the side in a very basic error. The punch can seem strong, with a satisfying snap of the gi. The position of the beginning and end of the punch are correct, but in the quick transition, an error has occurred.
Nor is this problem confined to the average black belt. Senior western practitioners can suffer from the same problem. Notice the arm position of the kata champion Milon (left), from his gold medal performance of Unsu, and Valdesi (right) in his instructional video on Bassai Dai.
Let's look now at some renowned Japanese karateka...