Back in the bad old days, people believed that fresh air and taking baths were unhealthy. They believed that evil spirits caused disease, that the brain had no function other than refrigeration, and a lot of other things that we now know are not correct. This is not to disparage people who lived in previous times (I’m an ardent medievalist myself); it is simply to point out that our understanding of science and the way things really are has changed with the years of experimentation and observation since.
Why do I take the time to point this out? Voice teachers and choir directors have been telling their students for decades to “support the tone,” “breathe from the diaphragm,” and many other ideas. The problem is that our understanding of physics and how the voice works has changed. Sound is a wave, and behaves like light (in as far as light does not behave like a particle). If we think about projecting an image on the screen, the light doesn’t have to work harder, or push more waves through. (I actually had a professional, respected voice teacher tell me that they couldn’t possibly damage their vocal cords because the sound was bypassing the vocal cords. The person was not willing to go with me to the cadaver lab to show me how sound bypassing the vocal cords was physically possible.) In the sense that sound behaves like light, we need to take the lessons we have learned in physics about light and apply that to producing sound. This is, again, not to criticize those who were following the best advice they learned, but to inform people that our understanding about voice production has changed dramatically.
As far as breathing from the diaphragm, well, it is impossible to take any breath at all without involving the diaphragm. The diaphragm is a smooth muscle, which means that it is not under our voluntary control. Instead, the diaphragm is encouraged to behave in certain ways by the use of the abdominal muscles. If you have enough air to produce a tone, that tone is already supported, by definition.
In short, unless you have specific knowledge of physics and how the body or other instrument works, please do not offer others amateur advice on singing or playing an instrument. Not only are you endangering others’ musical careers, and possibly subjecting them to injury by following your advice, but you are giving the actual professionals a difficult choice: either publicly correcting you, or exposing themselves to criminal and civil penalties, because the professional is expected to stop others from following false advice. (Take the example of a salesperson with a new product: “. . . and it contains three ounces of arsenic, which is proven to . . .”. A doctor who is present has the obligation to prevent others from taking this product.)
And please, don’t take advice from amateurs. Find a professional who keeps up with current anatomy and physics research.