Almost every martial art that includes punching has a straight punch that involves a corkscrewing motion of the hand, rotating from palm up to palm down for impact. I've been taught this in many different arts with pretty much the same reason being given. It increases power. I always wondered how a rotation of the striking limb could add any extra power to a punch. Some explained it as being the same as a bullet spinning, but anyone who knows anything about ballistics knows a bullet spins to keep it flying in as straight a line as possible. Rather than to increase power. In the same manner that a gyro stabiliser works by manipulating the Coriolis effect. The Coriolis force acts in a direction perpendicular to the rotation axis and to the velocity of the body in the rotating frame and is proportional to the object's speed in the rotating frame. A paper by Walker et al (1975) conducted a physics theory investigation in to a karate reverse punch. Considering only the masses and a associated forces of the arm. He calculated via masses and recorded arm speeds that his subject could generate 152 KN of force, of which, theoretically 0.05 KN could be attributed to the rotation assuming the arm was a solid cylinder and rotated a full 360 degrees. The writer concluded that this could be considered negligible. Now, I prefer to hit with a horizontal fist. I believe the arm/shoulder is in a stronger position with the palm facing the floor so a certain amount of rotation can occur if its not in that position when I start the punch but as I often have my palms facing forward the better to catch incoming punches there is usually little to no rotation. I have also noticed, while doing press ups in a muddy puddle, that rotating my arms near the top of the movement seemed to make that last portion a tad bit easier when your muscles are burning like fcuk! This lead me to the idea that the shoulder girdle is more efficient whilst doing that little rotation due to the increased involvement of the pectoral muscle and lateral deltoid.