A teacher's default response to a student's difficulty — solving the problem for them versus helping them build the capacity to solve similar problems themselves — has consequences that extend well past the immediate lesson. A highly directive teaching style can produce excellent short-term results while leaving students less equipped to handle setbacks independently, since the student never practiced the actual skill of working through difficulty without external rescue.
This is a subtler distinction than simply "good teacher versus bad teacher" — both approaches can produce technically skilled students in the near term. The difference shows up later, in how each group of students responds when a teacher or mentor isn't there to immediately resolve the next difficulty they encounter.
Renowned violin teacher Dorothy DeLay encouraged her students to see technical and interpretive difficulties as problems to actively work through, deliberately building their independence and resilience as musicians over time. Dweck contrasts her with Ivan Galamian, a more traditional and highly directive teacher whose students often produced excellent results under his close guidance but were, by comparison, less equipped to handle setbacks and creative problems independently once that guidance wasn't immediately available.