First of all, in my opinion, concentration mostly matters when it becomes a considerable portion of the effort - as in static apnea. Of course, there are those who claim that the sodium\potassium pumps in axonal membranes consume as much as 1\3 of the total energy the body usually spends, but this is untrue during intense muscular activity. In fact, while the blood supply to the brain remains constant under all but the most critical circumstances, I've noticed that the body automatically tends to reduce mental activity and conserve energy anyway.
Second, having something to do actively stimulates the both the motoric and the sensory cortex, as a runner watches his step and corrects it accordingly. Concentration is much easier when one has something to concentrate upon. Thus, unlike during statics, boredom has hard time activating itself and producing a 'screensaver' of idle thoughts.
Third, I would say that what matters most is not the fact that one is thinking, but rather, what exactly is one thinking. Most common thought which may visit an average, non-competitive, freediver's mind during a static exercise would be something to the extent of "I'm not going to break any records anyway. Does it really matter if I hit four or five minutes?". During active apnea, however, one may get a lot of parasitic thoughts revolving against one central idea: "I'm not gonna make it. I'm not fit for this. The doctor told me to be careful. I ate wrong stuff again. People on the street already give me weird glances." Such thoughts are bound to reduce performance. The truth is, most people consider themselves suffering from abnormal self-underestimatement of their capabilities without realizing that it is but an aspect of a self-preservation instinct, which at times happened to save lives when so called rational thought could not. An average person's mind and body can be compared to a stock PC - it is sold in maximum compatibility mode. After employing a few options which increase performance without any negative results, one has to tweak those system options which may potentially result in severe instability to further increase performance. The key is distinguishing between the pain one has to endure and the pain which could mean real harm, and not forgetting to value your own personal successes - no more of that "So I did it, so what? I'm still pathetic compared to..." attitude. Unlike most sports, I value freediving and related activities for NOT being a race (The PB threads seem a bit ruining the purpose to me), but rather a way to understand ourselves (And our environment, if you're inclined to say so) a little bit better.
- Levi.