Do as I say, not as I do
Seek to be yourself.
I have seen it many times. I bet you have too. A class full of students, all working hard, all trying their best. Most of them just want to get better at whatever it is that they are learning to do at that particular moment in time. Some however, whether they know it or not are also working hard to be "just like sensei". Now don't misunderstand me there is absolutely nothing wrong with any of us wanting to emulate someone we admire. As long as in doing so we do not lose sight of the fact that it is our unique character that makes each of us who we are.
Since the goal of any good instructor should always be to develop the character and talent that lies within each individual, and to bring out the best in them; when should we say to a student, " do as I say, not as I do"? For me the answer to that question really depends on a student's rank and their experience.
For students, up to the rank of 1st kyu. they should always adhere to proper technique, without any short cuts, especially in kata. For students who have held the rank of Shodan for at least a year a bit of technical latitude can be given, this will allow the student to experiment a bit as they try and find their way towards Nidan.
At Nidan less is more, now it is important for the student to practice more on their own, to make mistakes, to notice them, and to correct them on their own. Naturally you must continue to monitor their progress and to make comments, and adjustments when needed. But, if they are going to truly learn and progress in their karate they must now often do so on their own, and learn to separate what should be from what should never be.
At the rank of San Dan, however, freedom of action to develop their own technical style between movements should not only be tolerated it, should actually be encouraged. As this will help them to delve deeper into themselves and to open up a whole new aspect to their training and personal growth within the art of karate.
At Yondan a student's personal techniques are by now full fixed and integrated into every part of their being and every movement they make, they are on their own. As a teacher you can add bits here and there and tweak if needed, but in truth any real change is in all likelihood to late, that time has come and gone.
At Godan and above a Masters input on a course, or during a visit to the dojo will add to a students understanding of the subtleties of a technical movement or an application in a kata, but beyond that a sensei task ended long ago. They have found their own way.
So, by all means pass on all of your knowledge, and all your skills to all your students. Keep nothing back. Keep nothing hidden. But always at the appropriate time. Clearly in karate-do as in life knowledge is meant to be shared with those who seek it. What, when, and how that should be done is the real question.
Remember: "The object of any karate lesson must always be to develop the character of the individual, not to try and re-create the teacher".