It increases the force and depth of the tone waves.

The wider the pharynx is opened, without constraint, the fuller the resonance and the better the tone.
THE UNDER JAW The under jaw furnishes attachment for the muscles of the tongue and hyoid or tongue bone.

It also controls, owing to the connections of the larynx with the hyoid bone, the muscles that fix the position of the larynx.
The pterygoid muscles, which move the under jaw forward and backward, do not connect with the larynx, so their action does not compress that organ or in any way impede the action of the vocal apparatus.

A relaxed under jaw allows freer action of the vocal cords and ampler resonance.

The under jaw should drop little by little as the voice ascends the scale, thus opening the mouth slightly wider with each rise in the pitch of the tone.