There is this idea going around that if a person is an instructor or trainer or master of a particular school then they should also be some sort of high-level competitor, able to dish out the hard stuff to others in competitions. On the other hand there is this idea that those who cannot dish it out in competitions allow others to do so, and thus teach instead, hence "those who can't, teach". Both of these notions are fallacious in their own ways. Both need to be addressed so that we do not get the wrong idea and wrong expectations of our teachers.
Trainers cannot, in general, maintain the position as high-level competitors for a simple reason. A competitor's focus is on themselves: making sure that their skills are honed, making sure that their timing is in focus; and making sure that they are fit enough to compete in competitions. A person who truly takes on the position of the trainer does not have this focus. Their focus is on the individuals that they are training; focusing to make sure that the skills that they are teaching are up to date with the latest research; making sure that the method of delivery is understood by the students; making sure that the students can excel; the trainer's focus is outward rather than inward. Usually on more than one student.
Trainers spend much more time on others than they do on themselves. This is the reason why their skills may not be as sharp as the high-level competitors, because they have someone to think about other than themselves. Their students are their focus, not the next tournament win, not making themselves better through improving their fighting skills, but making themselves better through their teaching skills. This is another place where this idea of "those who can't, teach" is a misnomer.
A person who competes demonstrates their skill by performing it against an opponent. They can even sometimes give you all the technical information about the skills that they were using. Often, they cannot teach those skills. The trainer can. Not only does the trainer need to know how to perform the actions that the competitor does, but the trainer also needs to know how to teach those skills to others.
There are some great competitors who have striven to the height of their particular chosen pursuit, but they cannot teach. Just because you know the skills does not mean that you automatically know how to teach the skills. This is an additional level of learning which is involved. It is a process which the trainer goes through to learn how to teach, often the students learn through this process.
To teach a skill. The teacher has to know how the skill works. The teacher also needs to know why the skill works. For fencing, the teacher also needs to know when and where to apply the skill to a situation for it to work. These must be real understandings of how the skill works not just surface/physical understanding, but intellectual understanding of how the skill works. It is only through this level of knowledge that the teacher can explain to the student how to perform the skill properly.
If you see a trainer or teacher, and you see them fencing, examine what they are doing. You will notice that their fencing is more technical than athletic, because they are focused on getting things right. They may not keep up with the high-level competitors but fencing them will always be worth your while. They have a different focus, a focus which is toward the education of others in swordsmanship rather than seeking the heights of fame. It is an extremely important role as if there were no person willing to teach, there would be a lot less people to compete with.
"Those who cant, teach" is a misnomer.
Those who are teaching are doing, all of the time.
They may not be representative on the tournament honour's list. They may not be present on the world rankings. They are present in the school every week teaching people from beginners all the way up, spending their time on others. Without their technical knowledge and technical skill swordplay would not be what it is today. It is these people who do most of the study to ensure that what is being taught and thus what is being learnt makes sense, and works. Thank your teachers and trainers, they are a great asset.