When it comes to martial arts punches, the first image that comes to mind is that of the horizontal fist, perhaps with the first and second knuckles protruding. This is the most common version of the punch, popularized by the vast majority of Karate styles (and many other martial arts, for that matter) that practice it.
A lesser-known but very practical hand form for executing a punch is the 45-degree fist. The fist is not vertical, like the Wing Chun punch; nor is it horizontal, like the standard Karate punch. The fist is turned at a 45-degree angle. Hence, the name, of course. About 90% of the force is driven in with the first knuckle; and the remaining 10% carries over to the second.
This punching method has two distinct advantages. First and foremost, no matter what height the punch is executed to, it is the that first knuckle that gets driven into the target while still keeping the wrist strongly flexed. In the flat, horizontal punch, delivering the technique at a higher level results one of two things happening:
(1) either the wrist bends at an angle that makes it weaker, and therefore more vulnerable to a wrist lock;
(2) or the second knuckles of the fingers touch the target before the (intended) first two knuckles do.
Second, by keeping the fist in this 45-degree position, most of the major nerve points along the forearm are better protected. That equates to an opponent's block feeling less painful to receive.
Another key benefit of this punch is that it absorbs and distributes the impact of contact more evenly. Since the wrist, forearm and elbow are lined up in a straight line, the impact of the punch is cushioned by the forearm muscles and tendons rather than the bones. This, in turn, leads to less shock to the elbow.
Thus, the 45-degree punch is a very effective attacking technique for greater accuracy while keeping one's own wrist and forearms better protected during the course of an attack.