In every task, the subconscious and conscious mind have specific roles to play, and a confusion of these roles can lead to less-than-optimal results.
In ideal piano playing--where there is effortless flow-- the simplest way I can state it is this: the subconscious mind sends information up, the conscious mind "OK's" it. In other words, the conscious mind simply monitors, but does not judge or ask questions. (Is this a drastic simplification? Yes. But it does the trick.)
One of the distinctive mental features of performing under stress is that the conscious mind tends to kick into overdrive. It asks questions rapidly, tries to take over the playing process, and generally raises a big fuss. Unfortunately, the conscious mind has a very narrow bandwidth, and is actually unable to handle the incredible amount of information and physical tasks required in the playing of the piano.
Pianists usually take one of two approaches to this problem (I suggest both). First, they train themselves to focus on something other than the actual playing of the piece. Usually they focus on the sound they are creating (instead of, say, the hitting of particular notes). With training, this is a great technique. Of course the problem is that any lag in focus and the conscious mind jumps right back in to trouble-maker mode.
The second method is to train the conscious mind to be aware of every single aspect of playing before performing. This method subsumes techniques such as mental practice, ghosting practice, analyzing, knowing the piece in sections, etc. The idea here is to allow the conscious mind to be active in the playing process, but so thoroughly prepared that it isn't overwhelmed by hyper-activation and can, in fact, relax. (The analogy might be something like taking a test on material you know inside out.) Ironically, this conscious preparation allows the conscious mind the resume its role as (intelligent) monitor.
The next logical question of course is: OK but how do you know when the subconscious and conscious mind have been trained appropriately? After all, the big problem is that people play just fine in their living rooms, but have so much trouble in lessons and performances. How can they prepare for this correct mental functioning? And to that question I say: wait for part two.
To be continued . . .