Why does a boxer move his head backwards to minimize the effect of an oncoming punch?

Answer 1

You don't. If you do that, you set yourself up for whiplash like impacts. What they do, or at least when I was fighting in bite down hard on your mouthpiece and pretty much take the punch or try to angle it so it slides off you as a glancing blow. If your body moves back simultaneously with your head then it is obvious that you are trying to soften the impact of the punch, if you move your head back without your body there is something wrong, and you will destroy your neck.