Sometimes a singer will start an exercise as instructed, and then change something in the middle. I wait until the end of the exercise, and decide then whether to correct them. Sometimes no correction is needed, and sometimes they come up with something equally as effective or better. If they simplify what I asked for, I have to decide whether insisting on the original pattern has merit, or maybe I made it too complex.
It is always interesting to note the cognitive issues in such a dynamic leaning environment. It’s beneficial and enjoyable to strike a balance between familiar and novel exercises. When I teach teachers, and some advanced singers, I like to explain why I’m doing the next thing, or why I did the previous thing. There’s plenty of mystery in the act of making music; the vocal training itself should seem plain and sensible.