I make an effort to take as many shots to the head as possible. Or at least I did when i was younger.
why?
cuz in the street, thats where they are gonna punch, and i want to be able to take a good shot and not go down.
yes, you can train yourself to resist the knockout reflex
However, you CAN"T train yourself to avoid brain damage. Just because you don't get knocked out doesn't mean your brain sloching about in your skull is a good thing. As a former employee at a head trauma rehab center I can say that avoiding damage to ones brain is a good thing...deliberately taking excessive headshots as training is a questionable practice at best.
Peace,
Erik