It all depends on what you mean by "concentrate". Once you are down on the shot, the more you try to pay attention to every little thing that needs to happen, the less concentration you are using.
Once you decide where the cb needs to go, where it needs to be in order to pocket the ob and get position on the next ball, nothing else should be on your mind. As PJ posted, having a "trigger" that puts you in performance mode is a great technique.
The psr provides all the input you need -- visualizations, strategic options, logic, etc... It's all gathered through conscious effort. The actual body mechanics used to shoot the shot, however, are automatic. It's like a program ready to run, waiting in que for the start command. The program requires no amount of conscious effort. In tennis, for example, to return a lightening fast serve would be impossible if the player consciously focused or "concentrated" on any part of the mechanical process involved with swinging the racquet. So all the focus is directed toward one focal point, like the server's racket or the server's body or head. Nothing else matters. They focus on this with complete concentration, not letting anything else interfere, and the brain gets all the input it needs to automatically put the body and racquet in motion for the return. If the brain gets cluttered up with any other sensory input/data, like someone moving in the background, or a noise, or a feeling or an emotion, then it could affect the outcome.
So to "concentrate" is to ensure that no unwanted conscious inputs can influence the process that your subconscience mind is fully capable of pulling off on its own. This is where breathing comes in. Seeing what needs to happen, visually focusing on how it needs to happen, then letting your mind do the work uninterrupted by all distractions, can be accomplished by concentrating on your breathing. You are always concsciously looking/focusing on the shot, feeding your subconscious all the visual input it needs, but by concentrating on your breaths you keep your conscious mind from wondering about, where it has the potential to gather unnecessary inputs that may sabotage the subconscious mind's mechanical process.
Concentration takes practice. You have to lock in, visual focus on the shot, and at the same time you have to keep your conscious mind occupied (concentrated) on something that doesn't allow it to interfere with the mechanical process needed to actually shoot the shot. And it's tough. There will always be distractions. The key is to acknowledge all distractions and nip them in the bud before pulling the trigger, before sending the run command to that internal program waiting in que. The more you do this, the better you get at it. Eventually, you won't even notice most distractions. By concentrating on your normal breathing pattern, everything else will eventually seem normal, not distracting.