[jancsika]

It's difficult to say.

On the one hand I thing such lessons can be so effective because they are generally one-on-one. That already goes a long way to disseminating knowledge, especially when compared to the "knowledge dissemination theater" that is the large lecture hall.

On the other, musicians construct these insane pedagogical family trees. So-and-so's teacher was a student of so-and-so, who was a student of so-and-so, and now we're already back in the 19th century. My personal opinion is that a lot of that is more inspirational than practical. But it certainly has an effect.

There's also the easter egg that teaching can accelerate one's own mastery. It's basically forcing one to do rubber-duck debugging as a full-time job. Add to that an ever-expanding knowledge about the physical aspect of playing instruments (how to avoid injury, how to maximize practice time), and I think that's where a lot of the knowledge comes from.