Let's suppose you have a car, and you know how to drive it.
Does it mean that you also know how to teach driving, or how to fix the car if it's broken? The answer is - not necessarily.
You could be a very good driver, but when it comes to fixing the car, you probably would need a certified/professional car mechanic who specializes in technical issues of the matter.
In fact, when my child reached 16, she asked me to teach her how to drive, and pointed out that a lot of parents do exactly that.
My response to her was; My dear daughter, I definitely know how to drive, but you will not pay me a million dollars to teach you how to drive. I will leave it to a professional who would make sure that you will go on the road, won't kill anybody and, yourself, come back in one piece. Similarly, when somebody claims to be a vocal coach, it should not be assumed that this person also knows how to fix the voice issue/problem, if such occurs.
However, I have received quite a few obvious Voice Repair clients from various vocal coaches who were desperately trying to fix someone's voice, having no idea how to even approach it. Moreover, they used the conventional approach to voice mechanics, which is, in the first place, not very beneficial to a human's vocal anatomy.
The conventional vocal coaching suggests you to drop the jaw down and stick the stomach out, which could be very detrimental to the vocal health. In that instance, the voice gets drowned very low in its position and it pressures upon the components of the vocal anatomy and thus produces the strained vocal cords and abused larynx.
All of the above on its own could lead to growth on the vocal cords, (nodes, nodules, polyps), or even to a muscle tension dysphonia or spasmodic dysphonia. In my book, Vocal Science Flight to the Universe, I have a chapter which is called, How not to become a singer and work harder at doing it. That pretty much describes the 'methodics' of the conventional pedagogy, or the lack thereof.
So let's analyze this:
If the conventional approach to voice mechanics could actually be harmful to the human anatomy, how could it fix the already occurred vocal damage, which was caused, most likely, by that very approach or, perhaps, by the sufferer's own experimentation's?
Sounds like an oxymoron, doesn't it?
It does, as evidently, it is the fact and it is true.
So, whatever you do, please separate the ingredients, so to speak. If you need to learn something, find a teaching guru.
If you need to fix something, find the top mechanic and truly, it has to be a voice technician, who specializes in the field of VoiceRepair.