Breaking things — boards, bricks, concrete, bones — is about the delivery of kinetic energy. I'm not just talking about the cheesy pine board breaks they have 6-year olds do at the kiddie martial arts class, either. I'm talking about serious breaks, with real bricks, concrete, and so forth.
First of all, the relevant physics terms are in the mass times the square of the velocity portion of the kinetic energy formulation:
½ x M x V2
Mass (M) and velocity (V) are the user-controllable parameters that determine just how much kinetic energy will be delivered to the target. Let's consider the fist.
Generally speaking, the mass is the fist, plus, if the wrist and elbow are held rigid, the mass of the two arm sections and even the shoulder; real experts can hold the shoulder rigid at the time of impact, and so a surprising amount of central body mass can be counted in the formula. From there, breathing (and yelling) techniques that give you a rigid structure through your chest and stomach musculature, and the locking of the hips and legs through the moment of contact while (hopefully) sticking to the the place you're standing due to a good stance and decent traction so as to provide leverage.
But once you get that far, you're kind of stuck — because you don't have any more mass and you can't get any more leverage.*
*Actually, you can get a little bit more leverage by leaning into the strike; but that's very chancy because if you miss, you will be off-balance and that can be a significant opportunity for your opponent.
Velocity, though, both counts more (because it's squared) and is easier to boost as compared to your average other person, because we don't really generally work at anywhere near our potential speed.
Now, the speed would, you might think, be primarily dependent on the tricep and the chest — extending the forearm and straightening the upper arm. But again, you can do more. If you pivot at the waist, there is a vector addition to the speed from there. If your hips pivot against your feet, there's a vector addition from there as well. If you drive yourself forward with a leg (or both), that adds speed too. These things do add up, and you can improve them all. Do them all at once, get the vectors to all add together just as you go rigid... that's a fair speed bump. Squared. Multiplied by a good deal of mass.
The third factor is focusing the force. PBS once talked about "hitting with the fist." Bad Idea (note the caps!) Never hit with your two smallest knuckles. They're likely to break, for several reasons; if not them, then the supporting small bones in the hand. or both. You won't like it. There's even a name for it: "Boxer's fracture." So it's a bad idea for that reason (damage to you), but for another as well:
What you hit with is the front side of the first two knuckles. Why? Because the damage delivered to the target is highly dependent upon the size of the area you deliver it to. The smaller that area is, the more damage done. Technically, force per square inch goes up if force remains constant but the total square inches it is applied to are reduced. For example, ten pounds of force applied to one square inch is ten pounds per square inch. But that same ten pounds of force applied to ¼ square inch is 40 pounds per square inch. I like to use this more intuitive explanation with my students:
Suppose I take a book, weighing five pounds, and I set it carefully upon your head. Do you anticipate any damage? Students, if thinking, will respond "No." You should too. Okay. Now lets say I balance a pin, point down, on top of your head (because I'm just that good [dutiful laughter]), and then place the book's five pound weight upon the pin. Now do you anticipate damage? The answer, of course, is yes. Why?
Because although it's the same amount of force, loosely described as five pounds of downward pressure, it's applied to a much smaller area, which cannot possibly oppose it.
This is exactly why we train to hit with just two knuckleheads: Whatever force you can generate with your strike, the smaller the area you deliver it to, the harder it is for the target to deal with.
Some really advanced strikes use one knuckle, but that is extremely dangerous, because the energies are so high that the knuckle often fails — meaning, it breaks. I don't advise trying that without expert supervision, advice, and a huge amount of practical experience, deep self-knowledge of just how hard your knuckles are, not to mention the hardness of whatever you're going to hit, and... well, you get the idea.
The fourth factor, one that I usually don't go into great detail on, is a concept simply taught as strike through the object, rather than just "strike the object."
The reason is simple. When you strike, the motion has a beginning, peak, and end to the delivery of force. Maximum force, obviously, is not at the end when your limb is slowing down — it occurs at the peak, before you begin to slow down and unlock your body.
So you need to slow down and stop after you strike, not when you strike. Hence, "strike through."
These things: Speed; mass; tight focus on delivery; strike through... they are the keys to striking power that the woo-woo artists attribute to "chi" and similar nonsense. It's physics, no more, no less, and it's not that difficult to understand. But it isn't simplemended, it takes practice to get all those things working together, and it can be risky.
And yes, the same thing applies to the foot, elbow, knee, forehead... — any strike. Significant breaks are done with a small, hard contact area, moving at high speed, with as much mass as you can arrange in rigid connection at the point of, and through, the strike. It's risky. Don't try it at home. But it isn't magical. There's no such thing.