Our attention is limited. You might even say that attention is unary, that it's limited to one thing at a time. Intention, in an active sense, is certainly as limited as attention (since you can't form an intention to do something if you're not paying attention to it). Likewise, decisions and choices require attention, and aren't happening several at a time. How is it, then, that a keyboard player can play a piece of music that has more than one thing going on at a time? How is it possible to pay attention to all those parts at once? For a memorized piece, you might explain it by saying "it's all habit; it's automatic," but it's hard to extend this to explain how a keyboard player can sightread a piece of music with multiple parts.
Sight-reading multi-part music on a keyboard (or worse, keyboard and pedalboard) is a complex task, but it is possible, and it's even possible to explain, at least partly, how it is done. (It might even be possible to describe how one learns how to do it, but that's for another day.)
If you believe that a keyboard player magically learns to pay attention to many things at once and that, once the technique of having multiple consciousnesses is mastered, playing multi-part music is as easy as playing a single melodic line, except with more heads, you're in for a disappointment. It's not very much like that. And the process of learning how to do it is nothing like that.
What is it like? It is sort of like juggling. When you are juggling three balls, your attention shifts to whichever ball is in the air — the one you need to catch next. You aren't paying attention to all three balls all the time. Likewise, when you're playing keyboard music, your attention is shifting from one thing to another, while various "gesture trajectories" play themselves out, unattended.
Of course, this is only a crude analogy. Playing the piano is not very much like juggling — especially if you're sight-reading. To get into the nuts and bolts, let's consider some real examples. Here is the first of the "Little Canons" (opus 14) by Konrad M. Kunz (1812-1875) in its entirety (as you can see, "little" is no exaggeration):
If you're familiar with music notation, even a brief glance is enough for you to be able to answer most of these questions:
This is only a partial list; there are many patterns in the piece, many relationship between notes and between groups of notes; in all, the number of this is much greater than the number of notes in the piece. It may seem like this line of inquiry leads in the wrong direction: instead of having to notice the twenty-eight notes of the piece, you're having to notice several hundred relationships. But these relationships are the link between the notes of the piece and the strategies by which those notes are performed. ... INCOMPLETE
As an example: scratch your nose ... INCOMPLETE
From that knowledge, you could reconstruct the beginning of the score. The number of measures is larger than the magical number seven, so you might not have noticed that there are eight measures, and you might not have noticed how it stopped. ... INCOMPLETE
... the structure of strategic knowledge ... INCOMPLETE
... look ahead ... INCOMPLETE
... prepare ... INCOMPLETE
... initiate ...INCOMPLETE
... what are the time limits for short-term memory, trajectory motions, etc.? INCOMPLETE
... how does the strategy order change as the chunking level changes? INCOMPLETE
... what's the relationship between the form of what you learn in order to perform music and the form of the music? INCOMPLETE
... what reminds you what to do next? INCOMPLETE
... chunking, what elements, what scale? INCOMPLETE
... the role of guessing ... INCOMPLETE
When you're doing more than one thing at a time, it's like directing traffic ... things get confused until you work out all the possible interactions between them, and learn them ... INCOMPLETE