On one level, it’s advice that speaks for itself. But on another level, I think understanding it in more detail creates natural motivation to actually do it, and makes it easier to do.
The part of your brain that thinks is the conscious, and the part that doesn’t is the subconscious (not the unconscious--that’s what happens when you get punched too hard). The only real difference between them is ‘direct control’. You can directly control your conscious, but not your subconscious.
Here’s the rub--getting good at an instrument requires being good at a subconscious level. If you have to think really hard to do something, your training isn’t done yet. It needs to happen automatically. So you have to develop control over a part of your brain you don’t have direct control over.
It isn’t a paradox. You develop in-direct control. It’s possible, it’s just a different process.
It so happens that we were born with brains that work a certain way: if you think (that's the conscious) about a thing you want frequently enough, you tattoo a blueprint of it onto your subconscious. The subconscious, in turn, says “Ok, boss!”, and starts doing what it can to make it happen.
It's good to remember that "the subconscious" is still you, just the parts of you you're not paying attention to.
Terrible books like The Secret (*shudder*) will tell you that “The Universe”, or some mystical outside force will magically make your wishes come true if you think about them a lot and really believe. If you think that's dumb, then I think you're smart.
But it's based on a really insightful point that isn't dumb.
What’s really happening is that your subconscious does whatever was in your blueprint--that is, you start doing the thing you were thinking about without realizing it. Since it’s your subconscious doing it, you are, by definition, not aware of it when it happens. If you were aware of it, you would be doing it consciously. So to be fair to The Secret, it can really feel like “The Universe” is achieving your goals, almost on your behalf.
But that’s just what it’s like when you do something without realizing you’re doing it.
To be even more fair to The Secret, it is incommunicably easy to get distracted from thinking about what you want, and what to do to get it. So if you can whip up a cultish fervor in someone to such an extent that they start thinking about what they want all the time, they might well start acting differently and getting some great results.
But you don't need to develop really weird beliefs to think helpful thoughts consistently.