Relaxation is important for speed. Don't tighten your fist up until you're almost fully extended. It describes a failure to shift the weight off of one foot and onto the other when throwing a power punch. Barrel of a gun - Look down your punching arm like you're looking down the barrel of a gun. This will help that arm to provide cover for your chin on that side while you're punching.
Common mistake is for people to leave their chin open on the side of the arm they are punching with.
Depending on your personal style, it can also help to turn your thumbs downward to help bring the shoulders up and provide better cover. Your arms are like two soldiers guarding a fort. When one of them leaves the fort to make war, he has to build a wall to protect his post while he's gone. Also, in keeping with this analogy the other soldier at such times is extra vigilant. But, do not telegraph.