It is when the vowel gets stuck in a certain adjustment and does not change with the ascending or descending pitch.
It is like trying to sing a pure wide open [ah] all the way up; keeping it in the front as a feel. Or when the vowel is rounded at the top and one attempts to sing that same way in the middle of the voice. If one sings an [ah] vowel on a medium low note, there is a quality like the [ah] as in the word “father.” As we sing higher, the vowel should automatically shift to a more rounded [ah] as in the word “sorry.” Automatically. This vowel shifting depends on correct registrational adjustments.
The vowel is first physically formed in the vocal cords, with the cords taking a different shape for different vowels. That info is transmitted to the oro-pharynx, which mostly completes the shaping with some finishing touches by the front of the mouth, but this diminishes as the scale ascends. Therefore, yes, the vowel shifts or modifies, round or darkens as we sing up the scale. But, this is not something that we should be consciously doing, for we will do too much or too little; disturbing the working of the voice by not allowing the subconscious mind & autonomic nervous system to take control and accomplish the multiple minute adjustments needed.
Be assured, conscious direct control over the minutiae is impossible. It is like trying to walk up steps by consciously controlling, flexing and moving every muscle in the body involved with the “walking up the steps” process. Try it. It does not work.
Furthermore, this means that we cannot fix the voice through conscious and overt vowel modification, but must train the voice through the principles of registration, which is done through certain patterns of exercises that reflexively stimulate the vocal muscles. Correct movements, accompanied by a correct feel, appear in response.
We go through this process of letting go of pre-concepts and need for direct control, so that we experience the muscles showing us what they know. Then, we become educated about the right feelings and thoughts. We can give a conscious command and step out of the way; allowing the subconscious mind to finish the task.
To recap, as we ascend the scale, there is less length, less width, and less depth of vocal cords vibrating for the pitch. Our step gets taller and smaller, has a feeling of being more gathered up, and leans back. As the “registration” – the cords and muscles that adjust them – shifts so that we can sing higher, obeying the laws of physics, then the vowel that stands on the smaller pitch step must be smaller in size, as well as having a slightly different shape as the steps go back.
If the vowel gets sticky, we are in trouble. Some examples of trouble: if the vowel stays too big in size on the way up or too little on the way down, or if the vowel does not change its shape (pure to a little rounded as ascending and vice versa). The trouble is that the stuck vowel does not match the registration adjustment. Due to interconnection, there is a disturbance of all parts. Vowels, pitch step, consonants, breath flow are all out of whack.
However, when all is lined up, when the vowel shifts in response to a proper registrational shift, then all is right in the vocal world!