To rephrase a line from Sondheim's Into the Woods, "Careful the things you say, [singers] will listen." This also reminds me of the first agreement from Don Miguel Ruiz's book The Four Agreements, which is "Be impeccable with your word." I try to keep both of these things in mind at all times when I teach. Because of this, believe it is extremely important to stay continually educate myself and refine my knowledge, as well as the language I use with singers.
While I believe that nearly all voice teachers and choral directors have good intentions, I am not sure they are always sharing actual facts about the voice. The desired sound might be great, but the language used to encourage a certain sound can create misconceptions and technical problems. For example, the idea of tall choral vowels is something I deal with on a regular basis. I believe choral directors want a vowel that has a balance of brightness and darkness (chiaroscuro), and they probably want the singers to have a relaxed jaw and a low laryngeal position. What often results from just asking for tall vowels or darker vowels is a jaw position that is too open, an overly darkened sound, lack of resonance, a tongue that is pulled back, and overly rounded lips. If we take a little time to educate ourselves and use some knowledge of good vocal function, then we can use real language that benefits the individual singers and the overall choral sound.
"Breathe from the diaphragm" is another phrase I avoid. I beg my students to never tell anyone that I told them to breathe from their diaphragm. While it is true that you can not breathe without your diaphragm, a basic knowledge of anatomy tells us that we breathe from the LUNGS. The widely accepted inhalation for singers allows for expansion of the abdominal area, which is caused by the contraction of the diaphragm, along with release of the abdominal muscles. A better phrase might be "allow your belly to expand when you inhale." I do admit that's not as catchy as "breathe from the diaphragm," but it is certainly more accurate!
I recently heard some tenors talking about their high notes and how they can or can not sing in their chest voice up to a certain note. If they were actually using full chest voice as high as they said, they would be belting in a pretty unhealthy way. If you only know about chest voice and falsetto, then you might think that you only have those two options. I didn't fully understand this until I attended Jeanie LoVetri's Somatic Voicework Institute. The other option is mixed voice, and I believe that the homogeneous scale (simply put, all notes sound the same throughout a singer's range) that bel canto singing calls for requires mixed voice throughout the range. On low notes, the singer uses more chest voice and maybe a tiny bit of falsetto/head voice (thyroarytenoid & cricothyroid function for pedagogy people), and on high notes, the balance shifts in the opposite direction. In Pavarotti's famous recording of "Nessun Dorma," he uses a full range of mix, from mostly chest voice to mostly falsetto. The aria begins quite low, so there is a lot of chest voice. At 1:25, he begins nearly in falsetto and crescendos beautifully into a more chest dominant mix. Beginning at 2:08, the climax of the aria begins, and although it is strong, loud, and thrilling, he is still using a mixed production that varies from more chest dominant (modal or TA) on the lowest notes to more head dominant (falsetto or CT) on the highest notes.
I mention all of this because without this knowledge, if full chest voice and full falsetto are the only options, then Pavarotti's high notes would be described as "full chest." That would be much more difficult (and perhaps dangerous) to learn than the idea of mixed voice. Learning mixed voice is a priority in all of my teaching, and it enables students the freedom to sing whatever they need to sing (within reason). Regardless of age, students are able to grasp the concrete concepts of chest voice, falsetto, and mixed voice. Although I used an opera aria as an example, I fully believe that command of mixed voice serves all genres of music.
While my language may not always be perfect, and while some teachers might disagree with my ideas or prefer to use imagery, I strive for impeccable words in my teaching. Reading, listening, and discussing bring me closer to this goal, and I hope to pass that on to my students, as well as choirs and other groups of singers I have the honor to teach. As a result, progress is quicker, and everyone is happier!