She gives a pattern of feedback that makes the students practice well? In the sense that she gives positive feedback she functions more as a motivator than as a teacher. Her skill is teaching, it's only happenstance that she teaches music; has she taught shoe polishing or finger painting she would have produced the best shoe polishers and the most skilled finger painters.

Perhaps she doesn't have many complex skills but has strong fundamentals (think Tim Duncan of the NBA Spurs). She might make her students practice the fundamentals which will allow them to do more complex work as they get older.

Finally, she might have knowledge more advanced than her skill. She might not have the hand eye coordination or the processing speed to play sophisticated music but she might know how it's done. Imagine a 5 foot tall jewish guy that loves basketball. He's not gonna make the NBA. It's simply not gonna happen. However, he might understand the game better than many NBA players. Likewise he might be the best basketball coach in the world even though his athleticism (and hence his basketball playing skills) is less than that of NBA players. Likewise the teacher might have had a strong theoretical understanding but not have had the ability to put her theoretical knowledge into practice.

The first thing that comes to mind is maybe she's able to teach students how to practice more in their youth than she did.

That'd work at least.

I was thinking of Nadia Boulanger, with Astor Paizzolla as the distinguished pupil. Piazzolla was trying to be a classical composer, but Boulanger said his classical music was lifeless, it was his tangos that had fire.

Perhaps the multi-talented young pupil faces perilous choices about where to focus his energies. Whether the older great teacher can warn effectively against the common errors probably depends on having a breadth of experience, perhaps having put in many years as a mediocre teacher, following up pupils and noticing how things worked out. The teacher's own youthful errors might be uniformative even if severe.

Yeah, definitely surprising, but genius in any form is surprising.

There is an essay here by a former student that gives a sense of how she taught. And Philip Glass describes her as the decisive influence on him in this article, which also talks about her teaching a little.

Here's an interesting passage from Nadia Boulanger: A Life in Music:

It was Nadia's manner rather than her materials that was unique. Her intensity, her emotional involvement with her students, her broad knowledge of music in general, and her ability to project her own passionate enthusiasm for each detail as well as its over-all form, were the qualities that made her extraordinary. Her electric personality brought a distinctiveness to everything that Nadia did. In this, lies what one reviewer called "the difference between good teaching and great teaching," for in the latter "the student feels that the teaching enacts an extraordinarily intimate and demanding relation between the teacher and his subject, a relation such that the teacher's sense of his subject is indistinguishable from his sense of life."