Isshinryu karate emphasizes the use of the vertical fist punch; it is a trademark of the style. Here is my perspective as an ex-Isshinryu practitioner who now (occasionally) trains boxing and muay Thai for striking.
The spiel I gave when I taught Isshinryu was as follows: the vertical fist is part of a rising punch that:
- fits into the upside-down V shape of the sternum
- protects the wrist as the punch does so,
- allows the top two knuckles (of the forefinger and middle finger) to make contact instead of other, weaker knuckles, and
- is supposedly faster than a corkscrew punch
I have also heard other Isshinryu practitioners claim that such a hand position more efficiently transmits force to the fist.
Why It's Irrelevant for Most Practitioners
If one is training to get proficient in hitting people, the overriding concern one should have is achieving fluency in what Matt Thornton calls the "delivery system" of striking. Or as Daisetsu Suzuki put it:
Technical knowledge is not enough. One must transcend techniques so that the art becomes an artless art, growing out of the unconscious.
This means that we should evaluate training systems not by which one has a better checklist of reasons why they use a certain fist alignment (a comparison in which Isshinryu might win), but rather which one more consistently produces proficient strikers (a comparison in which boxing and kickboxing would win).
When choosing a style, it is dramatically more important to hit pads, work with the heavy bag, and spar with hard contact in a permissive ruleset than to worry about vertical versus horizontal fists. Avoid premature optimization--vertical versus non-vertical punches should be one of the last things to worry about in one's training. If a student was executing horizontally-oriented body punches, I wouldn't say that switching to a vertical fist was at all a priority.
That said, boxing does align the knuckles vertically during some punches--a lead hook comes to mind--but there is much less emphasis on this being a different kind of fist or a different style of punch.
Evaluation With These Considerations in Mind
I don't mind the straight vertical-fist punch for strikes to the body, but I don't see its usefulness for striking the head, and I don't think it's really that different from turning the fist over.
I'm no longer convinced by the idea that the vertical fist makes much of a difference for wrist stability; I would argue that if that is a concern for a trainee, that person should get stronger and hit the heavy bag very carefully and more frequently. I'm also not convinced that "fitting into the V of the sternum" matters very much. The xiphoid process might be somewhat easier to hit, but knocking the wind out of people seems more reliant on footwork, tactics, accuracy, and speed than on how the fist is positioned.
I flat out don't believe that the vertical fist is faster than other punches, or that one can't hit with the correct top two knuckles using non-vertical punches, or that it transmits force more efficiently than non-vertical fist positions. I just don't see any credible evidence for these claims, though I mention them because I've heard them.