One of the most distinguishing characteristics of Isshinryu is the vertical fist. Most new practitioners and disciples of other arts wonder why we make a fist in this manner, and why don't we twist or "corkscrew" our punches like most other styles. The answers are really fairly simple and quite well thought out.
For the answers, let's examine the dynamics of the Isshinryu punch. The properly executed Isshinryu 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 mostly quiet. The punch is targeted at the solar plexus, - not higher at the face or head. Striking with the first two knuckles of the hand and then snapped back - much like cracking a whip. At the completion of the punch the hand and arm are left in a position ready to punch or block again immediately with no wind up. When timed it is possible to land 3-4 of these punches in the time it takes to land one corkscrew punch, and if one believes the laws of physics i.e. Power=Speed x Mass it is easy to see that this punch is not only faster but more powerful that a twisting punch as well, as we are moving the same mass as in a twisting punch but with much more speed.
Biomechanically the punch/fist is also much more sound. Beginning at the fist and moving up the arm: The fist is made by holding the hand open and then slowly curling the fingers from the most distal knuckle until a fist tight enough to completely hide the fingernails is made. Then the thumb is pressed down on the second knuckle of the index finger. This makes an extremely hard and tight fist. Much less prone to injury, and a much more effective weapon.
Moving on to the wrist. Holding the fist vertically during the punch has the effect of distributing the impact to both the radius and ulna. Try a twisting punch and notice the position of the radius - especially when your target is on your opponents centerline - like the face or solar plexus. You will see that much more of the impact must be absorbed by the radius side of the joint where the joint is much "softer". Softer meaning that the joint on this side is comprised of small bones, cartilage and ligaments. Not to mention that the radius itself is by far the smaller, more fragile bone. The forearm is also left in a stronger blocking position. Blocking across the wide, muscular side of the arm instead of a single, exposed bone has obvious advantages.
Moving up the arm. In a twisting punch the elbow is turned outward, away from the body leaving it in probably it's most vulnerable position. When the joint is turned this direction and locked (as would happen if the punch was trapped, or slipped and countered) it takes little more than 20 pounds of pressure to dislocate or break the joint. Twisting the arm outward like this also has the effect of exposing the floating rib and in the case of a punch to the face or head also adducts the shoulder leaving it vulnerable to anterior dislocation and exposing a nice large vital/pressure point in the pit of the arm.
While this is a somewhat cerebral analysis of a simple punch it seems that most of this information is obvious to our subconscious minds. In thinking about this punch I realize that I rarely see even the most trained "corkscrew" puncher use one in a sparing match and even much less on the street. I think that one of Master Shimibuku's amazing talents was his ability to understand the state of Mu-Shin (no-mind) and to utilize it in a practical way. Imagine the power of knowing what the subconscious mind will do and training your body to be even better at it.