>When she does this with students, she asks them afterwards, “Was anything fuzzy?” It’s usually the part that they were messing up that’s fuzzy.

This is genius. Reminds me of the rule that if something feels fuzzy at the instrument it's always a conceptual error about the piece rather than poor coordination or poor finger strength.

>Why is it better to practice every day for an hour, instead of seven hours on one day of the week?

For the same reason that it's better to practice for 10 minutes several times per day instead of one hour straight.