If you punch with a fist properly, I assume you can break a person’s jaw or rib without damaging your fist. So, the basic question is, why isn’t force returned to you from the blow enough to break your fist if you’re breaking their bones? I know often you do break a hand when punching, so is there an ideal state of tension, pull back or follow through that prevents this while still delivering force?
Thanks.
I remember from karate class over a decade ago that when punching you make your fist as tight as you can and strike with your index and middle fingers’ knuckles. I don’t know if there is something more solid about the bones there and i’ve never gotten a chance to try it out. That’s just what the sensai told us.
If you hold your hand out limp in front of you, and try wiggling your knuckles (the ones at the base of each finger) up and down, you’ll find that the index and middle knuckles are rigid whereas the other two can move with respect to the carpals. In fact, there even seems to be a muscle which pushes the 5th metacarpal downwards. So my WAG is that the extra stiffness delivers more force by absorbing less shock.
This thread addresses the issues involved in the OP’s question. 
I too took Karate class quite a while ago, and I can give a sorta half-ass answer to your question.
A fully clenched fist has a ton of muscles supporting it, and a lot of bones. So it’s pretty dang sturdy.
Breaking somebody’s jaw, rib, or other similarly vulnerable body part would entail a certain amount of “give”. So when the target “breaks”, it absorbs a certain amount of the shock that would’ve gone to your fist.
We had these little plastic boards that were supposed to simulate certain numbers of 1-inch thick wooden boards that we could practice punching and kicking. The white one was one, the green one was two, the red one was three, etc.
I remember very distinctly the first time I decided I’d hit the green one. It hurt like hell, and of course, I didn’t break it. A couple of months later I tried it again, and when I hit it and it broke I felt no pain whatsoever.
Ok, so far this isn’t addressing what I meant to ask. So let me try again, this time leaving aside any references to karate or fists.
What is it about an object hitting another that transfers any damage to the object receiving the blow and not the … uh… “attacking” object?
“Damage” isn’t transfered from one to another. They both recieve the same force; one is simply more resistant to the impact/trauma than the other depending on where that shock is absorbed.