I was once stationed as the head of the criminal investigation division at a small naval base. Due to a shortage of manpower, the chief of police want to temporarily move investigators into the patrol division to act as patrolmen, and, when they were need, they would revert back to being investigators until the case was resolved. I was able to convince the chief that, instead of helping the department through the manpower shortage, this change would undermine the effectiveness of both divisions.
The physical and mental characteristics required of a patrolman are opposite from those required of an investigator. There are many differences between the two, but the main difference is that a patrolman is concerned with the macro, while an investigator is concerned with the micro. The patrolman is concerned with where a gunshot came from, and finding and arresting the person who fired the gun. The investigator is concerned with how the gun was fired, who it was fired at, the kind of gun it was, etc. An investigator in patrol would probably be shot in the back while studying the hole the first bullet made. A patrolman in investigations would probably never even think about the bullet hole, but would be chasing the shooter. A good patrolman may be trained to become a good investigator, but a good investigator cannot safely be a good patrolman again.
There are similar differences between a student and an instructor. A student is concerned with how he or she is performing a technique and how he or she may improve the technique. An instructor is concerned with how well all the students are performing a technique and how he or she may help the students improve their techniques. A student performs a technique as he or she thinks it should be performed, and usually thinks it was done well. An instructor must know the intricacies of a technique, be able to spot them in each student’s performance of a technique, and be able to tell each student how to improve the performance of the technique.
It is difficult for a student to become an instructor and it is difficult for an instructor to be a student. Students tend to have a micro outlook in a class; they are only concerned with their own performance. Instructors must have a macro outlook in a class; they must pay attention to the performance of all students in a class. When an instructor is a student in a class, he or she will be constantly evaluating the performance of fellow students and will have a difficult time concentrating on his or her own performance.
When sparring another student, a student is only concerned with being better at sparring than the opponent. When an instructor spars a student, the instructor also want to look good at sparring, but not to the detriment of the student. The instructor's job is show the student how to be better than the instructor. Instructors who show up or embarrass their students will soon have no students.
Therefore, it is difficult for a student to become an instructor, and it is even more difficult for an instructor to be a student again.