The voice is both physical and mental. A common fault heard is when the vowels are formed improperly, sometimes being swallowed but other times being crammed into the front.
When crammed into the front, the physical concerns the vowel being too far forward with too much “brightness” in the sound, especially as one ascends the scale, maybe even coupled with an overproduced smile. Oftentimes, this is accompanied by a mental concept that all needs to be “well forward” all the time. Of course, both the mental and physical are out of alignment in this scenario. The cords are too thick for the pitch. The vowel is distorted as there really is no such thing as a vowel sounding in the teeth or on the lips. The mind is confused as to what is true vocal reality.
True, on the lowest pitches, the vowels are more “down and in front” as a feel. However, they are never crammed in the face or the nose. They are “down and in front” in relation to the upper pitches. As one ascends the scale there is less cordal vibration, and the feel is as if the vowels travel back and up with the reverse happening on the way down the scale. This is what physically occurs when all is well.
The mental concepts match this as one thinks about, and sees with the mind’s eye, a pitch staircase with the vowel stepping up and down. On the lower pitches a thought of the vowel in front with the feel being under the hard palate is normal. We just don’t want to push the vowel into the nose, teeth, and lips area.
Pushing the voice anywhere is damaging, and too far front vowels are overdone. We certainly don’t want to try and hold the vowel there as we sing up the scale. All will suffer if this “vowel well forward” method is employed. The ill effects would be: inability of cords to adjust with the changing pitch, vowel distortion as they are not properly formed nor are able to adjust given the requirements of registration, and ultimately a loss of range.