Correct, sorry, I did mean brake with the bicep.
While you do want to be stiff at the point of impact, you shouldn't be focusing on it. From my own training, and for most people I've trained they start out stiff and push the punch, as once their fist is clenched they have muscle tension through the entire punch. They also decelerate as they come to the end of the punch as this is natural to protect the shoulder. This is slow, doesn't land against a trained fighter, and because it is slow it also doesn't do as much in the way of damage. It transfer more total energy during the punch, and if you ever get into brick breaking it works quit well, but it isn't a proper punch for hurting someone.
Once I get them to relax, they unclench the fist and relax the whole arm and "slap" with a relaxed fist. This is quick, but it isn't going to hurt anybody either because an unclenched hand absorbs the impact rather than transferring the force. If they clench just the fist throughout it will be better, but basically you get the energy of the fist transferred to the point of impact only. This will hurt, but it isn't full power. Full power comes when you contract the the arm, the core and drive with your leg and hip all at once to put the entire force of your body behind the punch. Basically everything needs to be rigid to transfer the power. But it needs to be for a small, small fraction of time, instantaneous if you can manage it.
Basically what I found fixed this quickly for most people was heavy bag and double end bag, and feeling the punch more like a boxer does. You will never land twice on a double end bag if your punch is too stiff and you push it, and you will feel it right away on the heavy bag if you are slapping without tensing as the bag won't move. If you just contract the fist properly, you can get a punch that will hurt, but you are just transferring the force of the acceleration of your fist. If you get that moment of rigidity in the arm, you get the force of your entire body. By and large it made me unpopular with most of my karate club as there is a rivalry with boxing, but whatever works.
There is no equivalent in the olympic lifts to that point of impact though. While you need to have your support muscles contract isometrically, they pretty much remain contracted through the lift. You aren't transferring energy in a momentary impulse, you are accelerating the weight with a constant force throughout the movement (if you can).
This was a problem I had bouncing back and forth between throwing and martial arts. I developed the technique for transferring at an impulse and ended up only adding force momentarily at the beginning or end of each movement rather than accelerating through the whole movement. A smooth pull where you accelerate throughout is much better for moving a weight, even quickly, then to try and just put the force into a momentary impulse. By applying force through the entire movement you transfer more total energy to the bar and can accelerate more weight faster. By transferring everything you have in your body in one instant in time, you cause more injury with a punch.