I think you are misunderstanding. We aren’t advocating sitting down and deriving the things on the fly during exams. We are advocating, essentially, studying that way. As a student, I found that when I really knew a topic, which generally meant knowing where equations came from and when and where they applied, I was one of the first students to finish an exam and I invariably did well. If I didn’t have that level of mastery, I was typically one of the last to finish and the result could go either direction depending on how good my memory was that day.