I believe the answer is: their growth mind-set.
The growth mind-set refers to the belief that all skills/abilities can be developed as long as people are willing to put in their effort and dedication.
Insulting the students when they have a bad test results could damage their confidence and discourage them to try to put more effort in the future. Praising their effort on the other hand would make the students feel good about it and make them more likely to put more effort in the future.
