ooooh, we are getting into sub-level concentration now. I actually studied this in college for a while.
I wish i could site some of this, but this is what i just remember.
The most efficient neural pathways do not take concentration to activate. They almost work on thier own. For example, how many times have you driven home without paying attention and then you realize you have no idea how you got home. Crud example, but it is basic.
If you can ingrain a set of commands well enough into your body, your primary conscience will not need to apply more than situation recognization to activate the commands. You see what is happening, and your CNS will take it from there. For me, bunnyhopping a bike is like that. If I think about what is happening, my hop suffers. I have to let my sub-level conscience just do its thing and execute the motion.
Lifting (or any repetitive motion) can "burn" itself into your neural makup, making the motions seem like nothing more than a begining and outcome. This is called Muscle Memory in some circles. It is merely a hardwire in your head for a memorized set of motions.
Now comes your question. Just as bignate said, the more weight you lift, the stronger you get. At first you will have to concentrate on where the weight is going, but after a while your body will take over allowing your mind to wander. You still have to realize what you are doing, and realize what the situation outcome will (or should) be. If you can harness the muscle memory and let your body just do the work for you, maximum gains can be had. How? Less energy is expended on analyzing the situation, and more funneled into actually doing it no questions asked.
I dont really think I answered your question, but basically your mind has to know what is going on fully, while letting the body handle the motions through muscle memory. It is the most efficient method for muscle to be recruited, allowing max power and strength.
At this point I could identify locations in the brain that handle the different tasks, as well as go into an arousal scale that predicts the best state of arousal for what you are doing. I dont think any of that would add weight to your squat, so I shall spare you of the banter.