When the aforementioned people are told to make fists with their extended hands and to keep their wrists straight, the first two knuckles of the fists are naturally in a straight line with the wrists and the forearms, therefore, when punching an object with the wrist straight, the first two knuckles will naturally strike first. Any force applied to the front of the knuckles will be transferred in straight line down the hand and through the wrist to the elbow and on to the shoulder. If the fist misses its target slightly so that the last three knuckles strike the target first, since the wrist is locked and straight, the line of force through the wrist has only a slight bend in it, so the off-center punch will probably have no adverse affect upon the wrist.
Ask any non-martial artist to push his or her hands against a wall. In which direction are the fingers and knuckles of the hands pointed? They are always pointed upward! This is not a behavior learned from training in a martial art; it is the natural way the body is constructed. To turn the hands so the fingers are pointed outward takes a conscious effort and it feels unnatural and awkward. If the same person were told to make tight fists and push against the wall, he or she would push with the knuckles upward in horizontal fists, not with the knuckles outward in vertical fists.