Action and reaction
Do you know why you jump when you see a snake even BEFORE or realizing there is a snake? Yeah, most of the time there is no snake, right? Better safe than sorry, right?
Well, if you had to see the snake, then analyze it, then decide it is a threat and then jump... too late mate!
Welcome to the amygdala:
These two olive sized brain structures are there to react for you. The conscious route goes through the eyes to the optic nerve on the back of the head, then to the front again to make a decision and then to the middle to the motor cortex. Too slow.
These to little friends will do it for you.
So, why is this remotely related to Karate?
I will tell you why: because thinking about how to react to a punch is a sure way to get punched, that is why.
Your reactions (block, then punch back) must be automatic to be useful.
And how to do that, you may ask.
Train your reactions thousands of times.
No easy way, mate.