Here is an absolute rule that is constantly broken. In an educational context, it is absolutely necessary when asking a student to do something to give him or her the tools required for accomplishing the task. Most often, this takes the form of a practicing strategy.
I was recently playing for a student. When we finished, the student said, "I just need to work on some stuff on my part."
"Like what?" I asked.
"Well, I think I just need to relax more when I play."
I recalled his teacher saying this to him in a lesson.
"OK," I said. "Great. How are you going to do that?"
The student was baffled. The teacher asked for something, but didn't give the student any tools for achieving the task.
"Relaxation always begins with the breath. Relaxation always comes from playing from the center of the body out to the extremeities. Tension comes from shallow breathing. Tension comes from playing from the extremities of the body detached from the center. Relaxed playing comes from knowing the musical goals and developing practicing strategies that allow the musical goals to come into focus. Tension comes from musical details expanding to roles of importance that impede the flow of goals. You need specific practicing strategies that will allow you to accomplish that."
Sadly, many young conductors fall into the same trap. They ask for something, but don't provide the necessary tools for performers to acheive the goals that they set. There is a careful line to walk here. You have to treat professionals in a slightly different manner. (More on that in the next post). With students, you need to provide tools.
It would be a mistake to think that I am only referring to a description of the technical apparatus necessary to achieve a sound. Some students respond better to imagery than a detailed technical description. The challenge of teaching and conducting is to find the thing that motivates your ensemble. You can try different things. What is vital is that when the goal is achieved, you must acknowledge it. You have to figure out a way to show the ensemble the sound that you want so that they hunger to acquire that same sonic beauty again.
Remember, you are teaching people to love beauty. In order to that, they have to experience it first. If you ask them to do something beautiful without teaching them how to do it, you aren't doing your job.