Why do theatre performers 'quiver' their mouths when doing vibrato?

Question

The classic things you learn from anywhere is that vibrato comes from healthy singing technique and good breath control. Singing gently, you can attain a nice vibrato without having to move your mouth at all.

But when I watch theatre performers sing, they 'quiver' their mouths almost as if it helps produce the vibrato. But these are professional Broadway-level performers so they are obviously not producing it artificially. I've not heard of this technique before. Is this something exclusive to theatre?

Answer 1

I usually only see a 'mouth quiver' if someone is singing with too much tension or trying to produce too big a sound without the proper technique to back it up, OR having to sing a demanding role in several performances weekly without time in between to recover or trying to perform when you're sick and should be on vocal rest.

Answer 2

Jawbrato or Gospel Jaw

...describes a method of simulating vibrato by rapidly quivering the jaw and tongue. This movement creates rapid changes in tone and in vowel formation, leaving the listener with the impression that the singer is creating vibrato. However, the vibrato that is produced by moving the jaw or head is generally not very natural sounding, and the singer looks tense and silly while singing this way.

The linked article describes, at length, several proper and improper ways to produce vibrato.