To get us ready for the discussion on squillo, or the second layer of the vowel for the tenor, I would invite you to listen to two lions from the Italian school who exemplify beautifully the concepts I have been talking about: Mario Filippeschi and Rosanna Carteri (female with squillo anyone?).
Filippeschi focuses very much on keeping the path of the breath unobstructed, but he never pushes to get squillo. It is interesting to listen how the forward tongue makes his sound dark while maintaining artistry and gentleness. His key? Closed vocal folds, cry phonation, but no tongue used as a stopper to pressurize the cords. Result? Dark voice with extreme bright center. Without the squillo, the cavita’, or the expansion of the darker harmonics, would jeopardize the quality and ease of the voice.
Nicola Maritnucci well stated that cavita’ without mask resonance (maschera) “spacca la voce” – breaks the voice. The correct closure of the cords increases the power of your voice, but without pushing. All you have to do is keep the mental intention of the intensity of the first layer and let the harmonics expand. The closed cords set up the right balance. Just don’t push breath against those cords.