Actually I should edit my previous statement, to change all zazen with shikantaza. Because that statement is made with shikantaza in my mind as the OP.
If we are discussing normal zazen with breathing, definitely concentration is required.
However, hopefully it doesn't give the impression that in shikantaza there is no concentration.
There is a big difference between
Concentration without concentration, and concentration with concentration.
The first one is you naturally concentrate, and it is always there. Because it is always there, there is no such thing we increase the concentration or my concentration is decreasing so I add it more.
This concentration is effortless because we know precisely that actually we are always in concentration mode. If we see ourself, actually our concentration is very good. Almost everyday we are concentrating. Even in day dream, we are actually concentrating, we are concentrating on the content of that daydream.
The problem with our normal concentration is te object of concentration is shifting between one self to another self. Although there is no self, because we don't know that the concentration that we are having is the concentration with that mistake.
But, once we can see the emptiness of self, you can see that there is no way this reality in its actual mode can run away from this emptiness. It is always there. Even wen we think it has a self, this reality is still running in the selfless mode.
Because we can see that in whatever condition, whether during we aware of this ad during we don't aware of this, everything will always running in selfless mode, it really give us a shocking question: What am I doing? What am I focusing?
Since everything is always selfless, even we are concentrating on the breathing, in reality we are actually doesn't have focus. But, we think we have the focus.
By knowing that we are actually in really cannot have any focus, naturally we will always notice this selflessness. Because this monkey mind always active, now this monkey mind will be very active in concentrating of selflessness.
It bring us to the next step, if there is no focus, how can there is concentration?
So, it bring us to the state where there is really no effort or no intention at all to focus, because we know exactly there is no concentration due to selflessness, but at the same time this concentration is there naturally.
It is really a meditation without a meditation.
We cannot understand this with logic, because if following logic the statement of meditation without meditation is the logically flaw statement. Logically, someone who is impair in their logic will say this statement and that people is normally classified as crazy.
But, if we don't try to understand that by thinking but by doing it, we can actually know it.
I also don't know it by the way,
I also don't know what I am talking about.