The Tongue Bone is NOT connected to the Jaw Bone!
Singers! Separating the tongue and jaw to free tension and tightness of sound and instrument. Many singers try to prevent vocal failings: cracking, cord separation, etc. by jutting the jaw forward. This jaw lock will keep the sound overly “chesty” and will not allow “flip” into the pharyngeals and more head based anchor. The jutting jaw also raises the larynx, and while the tongue and swallowing muscles become attached to tension, the larynx raises even more. When approaching the upper middle and high of your voice - try to keep the jaw slightly - very slightly - back. The back is really in direct relation (reverse psychology) to not forward. The jaw back, is similar to a clarinetist’s embouchure. The jaw will sit ever so slightly back, and also, yes, the jaw needs to swing down, but excessive opening of the mouth can spread the sound and also bring too much “chest” up. An overly open jaw will create an overly closed throat. The opening this overly wide and jutted forward jaw needs to translate to a more internal quality where the pharynx, soft palate, pillars of fauces, and larynx create more inner space. I like to think of it as a Muppet head. Position your hand like a puppet now move the fingers more and the thumb less. Now, put your fingers on your chin and try to move the upper jaw with out so much movement from where your fingers on the chin are placed. The top jaw will create space and the lower jaw will not move as much. (Look at Joan Sutherland). Many singers think the jaw will help with the belt. A large mouth cavity may help, slightly, with some lower tones but once above and ascending into more of the Mix-Belt the jaw must be free of tension, and the hard palate and pharyngeals take over. And of course when in head voice the jaw aperture should help aid the sound not so in the mouth, but above — the jutted tense jaw will not allow this. Try to separate the tongue movement from the jaw (and also larynx) by doing exercises of just ee-oo ee-oo, and ay-oh ay-oh (one or two notes i.e. unison or major second) with your fingers placed on the chin. The tongue should move but the chin/jaw and larynx do not need to move, a lot. When not working for pronunciation the tongue should stay at the base of the lower teeth, and sometimes press on the lower gums slightly for slight tilt, brilliance, and more open pharynx. The words and emotions cannot get overly placed in the jaw. The idea of emotions and pathos, etc. colored in the jaw -- will not serve you. Again, experiment with a slight back feel to the jaw — SLIGHT. You will have more resonance, and less tension i.e. more ease, beauty, and longevity! Enjoy!