Overtime it strengthens them to an extent. At the moment of impact you have to tighten your fist, and tense your forearm to keep your wrist straight. There's an axiom, "There's no wrists in boxing." This simple wisdom just applies to punches. You always have to keep your knuckles and wrist in line with your forearm when you punch.

It's also important to have the proper alignment to your shots and the right distance in between your feet according to the distance between you and your opponent. This has to do with leverage and simple body mechanics. As for power a lot of it due to your feet, because you have to shift weight from one leg to another to generate any power. Bad technique can stop you from thowing hard punches. A common one is that people whom wing their punches lose power through their shoulders, which can also account for shoulder injuries.

Good technique coupled with a strength is the most efficient way in generating punching power. Forearms strength and grip strength go hand in hand. If not just for looks perhaps this is why Bruce Lee trained his forearms so religiously.

I should also note that to prevent injuries you should have your bag hang to where the bottom of it is at waist level. The swinging helps, it's detrimental to your joints/tendons to hit something that doesn't move.
Quote Originally Posted by greynotsoold
That swinging etc.. i exactly why one hits (or should be hitting) the heavy bag. In old tapes of Marciano, Mickey Walker, evey one I can recall, the bag swung a lot. (The exception being Liston for the first Ali fight; his trainer held the bag so the press would see how hard Sonny hit and not how shot his legs were.
That motion is how you learn a number of things, among them how to move your feet to position your sefl to hit an opponent that is not wanting to be hit, how to punch fast and hard when in position, how to set up particular punches and how to handle a body when fighting in close how to sidestep and step around with minimum movement. You learn to contrl the bas sway with your jab, how to step in andpunch inside and how to punch your way back to long range.
Your bag should swing and spin- I'm not talking about it getting parallel to the florr or like that- and don't hang it too low. Its good if the bottom of the bag is @ at your waist; its good for landing the halfhook/halfuppercuts that you throw to the body and dosn't hurt your wrist as readily. Also, you don't want to be landing punches real close to the top of the bag, as it seem the chain ...well the bag doesn't move as readily and I've always thought that the way it does move lends itself to hand and wrist injuries.
Don't overtrain, or work to intensely some people get carried away with what they are doing and keep going if left to their own devices. Keep true to the technique, and don't work on the bag when you are very fatigued. It isn't necessary to hit the bag everyday.

I think most heavybag's nowadays are too hard which is also detrimental to your joints and generating punching power. Taking some of the stuffing out helps. Make sure you treat the bag like you would a fight, you work in rounds, and treat it like an opponent, incorporating sidesteps, moving out of range, feints, setting up your shots and defense. Just because it is useful in developing punching power doesn't mean its the point of hitting it. Do everthing for a reason.