I am honored to be asked by Jack Livigni to address this issue on his blog. This article first appeared a few days ago on TenorTalk, here.
There is a kind of brightness that is accompanied by the back of the tongue pressing into the back of the buccal pharynx. Some singers have found comfort in this sensation and believe erroneously that this adjustment is necessary for the production of squillo.
It is important to understand the order in which the brightness and tongue pressure occur. First, the tongue is the most flexible muscle in the body. It is a multi-layered muscle, the parts of which are able to move independently of one another. Like the diaphragm that responds spontaneously to provide pressurization when it lacks, the tongue often spontaneously responds to make resonance adjustments when the source tone and the vocal tract vibrations do not agree. The tongue, in a sense, is the primary instrument of resonance. Its subtle or sometimes not subtle movements have a powerful impact on the resonance of the vocal tract. Indeed what we refer to as tongue tension in singing is a compensatory reaction to a resonance imbalance.
Therefore the erroneous process of brightness accompanied by a depressed tongue occurs as follows: when the source tone is pressed (and a pressed tone requires less fold mass in order to prevent pitch from lowering), the sub-glottal pressure will rise to the point of pushing the larynx up out of its naturally low position, causing a rise in the frequency of the first formant (a narrowing of the lower pharyngeal space raises the frequency of the first formant). In that situation, the tongue comes into play, either to push the larynx down to achieve a lower first formant frequency or else simply push into the pharynx to narrow the space even more and raise the first formant further. In the latter case, the lower overtones are lost, robbing the voice of natural warmth. The resultant sound is extremely bright (strident). Some mistake this extremely bright sound for squillo, which it is not. Such a thin sound does not carry in the hall very well. It may be heard better than a sound lacking in medial closure (the kind of hollow sound typical of some basses) but it will not carry well enough to have a real impact when the orchestra is involved.
Where this causes problems for the tenor (or any voice type for that matter) is in the passaggio and higher, where the first formant needs to lower to allow the second formant to take over. In such a circumstance, the singer will have a difficult time “turning the voice” (girare). The best structure for a squillo is a source tone that allows both adequate closure (reinforcing the epilaryngeal narrowing) and adequate flow, preventing the larynx from being pressed up from excessive sub-glottal pressure. A tone with adequate fold mass (right amount of chest content, i.e. vocalis activity) makes just that kind of structure. In any case, the retraction of the tongue is ill-advised for the reasons mentioned. Correcting the source tone by having adequate fold mass will prevent the necessity of pressing (greater fold mass increases the length of the close phase, as does pressing). The only caveat is that one must be careful not to sing with too much mass. The balancing factors are mass and closure. They should keep each other in check.
In more simplistic terms, the glottal squeeze accompanied with retracted tongue gives the impression of chiaroscuro. In a sense there are elements of bright and dark in that kind of production. However, the bright is too bright and the dark is a compensatory mechanism to cover the inherent stridency of the source tone. Furthermore, the retracted tongue prevents the production of intelligible vowels because it is stuck making up for an imbalance in the source tone and therefore the resonance of the vocal tract. At the extreme, the retracted tongue could push on the epiglottis and obstruct the natural propagation of air. The sound would become seriously muffled and would be unviable. In short, the glottal squeeze accompanied by tongue retraction should be avoided.