Arguments For and Against
The following topics discuss horizontal and vertical punches and, since twisting or the lack of twisting is a part of the performing the punches, it is included in the discussion.
Vertical punchers say the twist causes a weak wrist. Some vertical punchers say that when an opponent moves in as you are punching, a twisting horizontal punch may buckle, and the wrist may be more damaged than the target. However, in reality, the opponent is always moving in one direction or another; opponents do not stand stationary and get hit. Twist punchers train to punch from any variety of ranges and movements. In 30 years of martial arts experience, I have never seen anyone injure a wrist because of twisting a punch. I have seen wrists injured because they were not locked, but that would also be a problem when vertical punching.
Vertical punchers say their thumb position is stronger. Some vertical punchers say their punch is stronger because the thumb is placed on top of the fist instead of folding under the fingers, thus making the wrist stronger. If this were true, then any gain in wrist strength is offset by a loss in fist stability. Try it yourself, make a fist with the thumb on top, and then squeeze the fist as tight and solid as you can. Then try the same thing with the thumb folded underneath in a natural position. Which fist feels more solid? If the thumb on top would prevent injuries to professional fighters, they would use it. Boxers fold their thumbs underneath, for fist stability and because they worry about thumbing (the thumb poking into an opponent's eye) and the thumb being snagged and sprained by being pulled backward. When free-sparring, a slack thumb on top of a fist may lead to an accidental thumbing of the opponent or the thumb being snagged on the opponent's sleeve or grabbed. In reality, wrist strength is not affected significantly by either of the thumb positions. As long as the wrist is held straight and locked, it will not be injured in a punch. The wrist is strengthened by punching, so years of punching with either method will strengthen it to perform the desired punching method.
Vertical punchers say their punch is faster. Some vertical punchers say their punch is faster because there is no twist. If we assume this is true, how much faster is it? Sparring is not drag racing or downhill ski racing, it not judged by thousandths of a second. A few hundredths of a second in speed will not make any difference in whether a punch is blocked or not blocked so the hand is quicker than the eye. Once you see a fist move, it is too late to block or avoid it. To block or avoid a punch, you must detect the punch before it moves by reading the opponent's body language. Therefore, a punch that strikes harder will be more effective than one that is a millisecond faster.