Particularly thinking about simple Kinetic Energy, and then thought I'd write it out, and put it here to see if it's sensible, and for welcome input.
Ok, we've seen martial artists break bricks (and things). They might suggest various things surrounding the concept of chi, or ki, and some maybe be fraudsters*, but I know that if I punch something, it hurts, and there are in fact people that can put an incredible amount of force into hard things, and not destroy their hands as might be expected.
K, so the thought is that they really want to break the brick. If they fail, and the brick remains solid, then all of the "action" that they are putting into the brick will "react" upon their hand.
If, however, they produce enough force so that at the very moment of contact with the material, it breaks, (I don't know what you'd call the force required to fracture a brick, something along the lines of coefficients of friction, but I know that's not it), then once the initial breakage happens, then the martial artist is no longer up against the solid brick, but is instead simply pushing pieces of brick out of the way. Thus, the "reaction" to the hand is minimized, and the Momentum of the punch is simply released as that of the attack's follow-through.
So, it seems that the trick is to IMMEDIATELY break the brick. And if you don't believe that you can do it, then you'd better not, because you simply have to be committed to breaking it.
As an aside, I'd guess that this might be more along the lines of where chi would come in, because Chi is more about how the martial artist is dealing with his own internal workings. For example, in order to break a brick, you need a very rapid shock, and not a slowly-released impulse. The martial artist has to have that focus (and training) so that his or her hand is a metal hammer, not a sponge. So, the martial artist is typically using chi, or ki, or whatever their cultural equivalent, to give them the focus such that the maximum amount of energy is transferred from their fist to the target in a powerful burst, which I suggest might revolve around the practitioner's more highly developed control of their muscular and nervous systems.
* Even credible martial artists use some tricks, like putting bricks on spacers so that they're not trying to break through an effectively (almost) single giant mass of stuff.