It's a bit like learning to drive - to begin with you have to think consciously about everything you're doing (which means thinking about three or four things at once, which is enough to make anyone nervous). With practice you can do the job of driving without thinking about it, and use your conscious attention to think about driving well (and in the right direction).

Which is also, perhaps, why "Craigie" had his/her confidence knocked so badly - if you were confident that your voice would go where you told it without having to think about it, it would be pretty nasty to be told that your voice wasn't going where you thought it was.