This post is about one of those technical things that I realized a little while ago but isn't spoken about a whole lot. I'm not sure if that's because it's incredibly obvious to everybody but me, or if it's spoken about a lot in areas I haven't looked, or if it's kind of esoteric. Regardless, I'll post about it. If this is absurdly obvious to everybody just post in comments how much of your time I've wasted.
Boxers, along with certain martial arts (systema comes to mind), have a very different method for throwing punches than karate does. Look particularly at the shoulder. A boxer is trained to raise the shoulder while punching - hunching the shoulder up towards the ear - to protect the jawline while punching. You can see this taught on instructional DVD's, in books, and you can watch it in action in boxing matches. The elbow is also lifted (this depends on the punch) - it points to the side or even slightly up (the way your arm turns when you're using a screwdriver to take out a screw - elbow pointed up slightly, shoulder rising) instead of down.
The traditional karate punch, on the other hand, is executed with the shoulder packed down (the lat tensed to keep the shoulder down in its socket), the elbow pointing down or mostly down, not up. If you want to see this beautifully demonstrated, check out an iconic picture of Mas Oyama punching out a candle flame - his shoulders are pinned down, the chin tucked. It's the way your arm would rotate to drive in a screw.
Why the difference in mechanics? I think we can trace it back to the kinetics of a punch according to the two disciplines. In boxing the image is of a thrown punch - the first/ forearm unit is hurled at the opponent. At impact the damage is done by the fast moving fist - not by the torso or hips. The hips twist at the beginning of the punch, to give it more initial velocity, not at the finish, to drive it through the opponent. The punch is launched. In karate, on the other hand, the punch is driven through the target. At impact, the elbow, shoulder, and core tighten dramatically, dirrecting the rotational energy from the torso into (and through) the target. The fist/forearm complex isn't a launched missile - it's the tip of a spear.
If you doubt that the karate mechanics make a stronger connection between the fist and the hips, it's easy to test. Stand up with a partner. Hold your hand out in a fist like an extended punch. Have your partner push back against your fist. First try it with your shoulder up by your ear and your elbow pointed slightly upwards. Next, repeat, but this time pack the shoulder down and keep your elbow pointed down. I can guarantee your arm will collapse under much less pressure with the shoulder raised and elbow up.
How did this difference occur historically? I'm not sure, but part of the motivation behind the karate mechanics is the history of training for punches on a makiwara, instead of a heavy bag. The makiwara is like a spring - you hit it, and it bends and springs back. If you hit it hard but don't sustain the pressure after the impact the makiwara will knock you backwards. A heavy bag, on the other hand, just absorbs impact. It won't spring back at you (it might swing away and swing back, but people don't hold their fist against the bag long enough for that to knock them on their butts). If you need that kind of sustained force following your punch you'll never get it with boxing mechanics. If all you're concerned about is the impact, then that's another story.
Which set of mechanics is better? That's a question I wrestle with. Figuring out which leads to harder hits isn't straightforward. The physics are beyond me - too many variables I can't quite figure out. Looking for hard hitters doesn't help. Many of the hardest hitters in sports (Fedor Emelianenko, anyboy?) use boxing mechanics - but that might be a result of glove use or just the fact that most sport fighters study boxing, and they would hit even harder using karate mechanics. Karate mechanics seem safer for the shoulder joint (which is strongest when packed, instead of raised). Bruce Lee used karate mechanics for his punching.
Take home point: you should at the very least be aware of the fact that there are at least two competing views of how to punch properly. If you're reading books on technique or studying fighters, you can't always apply their advice to what you're doing. If you try to punch both ways simultaneously the result won't be pretty. If you can't decide, go with whatever your style teaches. I train the karate style - I suspect it's better, but I can't work out enough of the math to be certain.
To do the "karate mechanics" punch, tense the lat so your shoulder stays down - imagine you're trying to keep your shoulderblades (scapulae) in your back pocket. I like to imagine squeezing the punch out of my armpit. Your hips don't twist at the start, to launch the punch, they twist during and at the end of the punch, to transmit the kinetic energy of the torso into the target. If your hips twist at the start then by the time the punch lands your torso isn't moving anymore - which means the energy is lost.
Caveat: I call the packed shoulder - elbow down - body as one unit - method the "karate method" - I don't mean to say that all karate styles must punch that way or that it's more authentic. It's simply the method taught in the karate styles I'm familar with. If your style teaches the other style, I have no problem with that.