I agree with ed-swfck and others on this, but would like to expand a bit. Using the bottom three knuckles for any punch is both a safety feature for the striker, and a structural improvement for the punch. Jab, cross, hook, upprcut and vertical punches all make use of this structure. As Ed stated, this allows for an alignment with the forearm bones, locking the wrist into a more stable position. If you hold your arm straight out while making a fist, you may notice that the pinky, ring and, middle finger knuckles line up with the wrist and forearm, making a straight line from your knuckles to elbow. The index finger is not aligned with your forearm at all. Now, turn your wrist so that the index and middle knuckles are aligned with your forearm. What happens? Your wrist is now bent and you may feel a torque in your muscles that requires more energy to hold. Now, do this test on the floor. Do a knuckle pushup on the pinky/ring/middle knuckles and pay attention to how it feels, even if you just hold yourself up on them, you can do so easily. Now, hold yourself up on the index and middle knuckles, then do a push up too. How's it feel? Shaky/wobbly? Given time you can make this structure work, but I'd much rather utilize the naturally strong structures of my body within an hour or two of training rather than have to build up new muscle and nerve connections that are not as natural and require more time to make functional. The three knuckle alignment is more natural and structurally sound than the given two knuckle alternative. It is safer for the hand, which is not a natural weapon, to align the fist such that the striking surface is the Pinky/Ring/Middle knuckles, again, locking the wrist in a straight position, and alinging it with the forearm. This is very true for bare handed punching, as in a self-defense situation, you'll not likely have time to put on the gloves. Now, when you're wearing gloves, especially boxing gloxes, this alignment is not too important to consider, given that you've got padding on, and maybe a taped wrist and hand. I'm not saying that people don't think of this, just that when wearing gloves, you can do more with your hand and be safe about it. Being bare handed in an encounter, it is best to have analyzed and trained this detail so that you don't do your attacker a favor by hurting your hand for them when you do hit them. Kind of detrimental, no?
