Avoid lapse in attentiveness

The other day my student Brian Palmer and I were sparring for about 20 minutes. Afterward Brian, who is one of the smartest people I know (and therefore, I always listen to his comments carefully), told me that he feels that karate is a great way to learn to keep his attention and focus, when we spar for 20 minutes, the moment he has lapse in his attention, that is when he will make a mistake and I would get him.
In karate, when sparring, more than anything else I know, if you lose attention, you get immediate feedback.

But to keep your attention for 20 minutes or more it cannot be forced, it cannot be an attention with effort, than one will get exhausted and momentarily lose of focus is inevitable.

Focus has to be relaxed and effortless, I am just “present” and my body will do what it needs to do.

How do we do that? How do we develop that kind of attentive, present mind?

We say “as if your eyes are behind you over viewing”, “don’t use eyes”, bypass the brain”, “don’t look to opponent’s outside (details), but rather look to the breath and eventually to the heart”, “allow the feet to catch the rhythm and control own movement, feet are the boss”.
All those directions over time will allow us not only to see the small details without lose of the whole, but also to keep our attention for long periods, since we spend minimum amount of mental energy.