Why is it that particularly difficult to crescendo-diminuendo on a note that lies in the muscular passaggio (where the mechanism goes from a vocalis-dominant [thickening] mode to a crico-thyroid-dominant [stretching] mode?The reasoning here is that during the crescendo phase, more sub-glottal pressure is exerted on the vocal folds. If the phonation is truly balanced and the set-up is strong enough to handle the breath pressure, the crescendo occurs without a glitch. However is one of the two muscle groups is weak, it will buckle under pressure. That is, if CT is weak, the voice might lower slightly during crescendo. If Vocalis is weak, the voice might go sharp during crescendo.
If the set-up of the particular pitch (Fundamental Frequency) is unbalanced, say that one of the two main muscle groups is hyperactive, the Inter-arytenoids will compensate. In the case of too much vocalis activity, which would lengthen the vibration cycle, the IAs relax to accelerate the "opening" portion of the close phase. This relaxation might cause some breathiness or else the folds may just fall apart from each other. If on the other hand the CT is hyperactive, it could cause sharpening, the singer with a good ear might compensate by slowing down the opening phase through IA contraction (in essence pressing the fold edges together). In such a case, the sub-glottal pressure might be too much and result in a sudden break (cracking). Alternatively, the folds may endure the pressure but the tone would sound tight.
Of course, any imbalance during the crescendo will be experienced badly during the diminuendo. If the tone is pressed during crescendo, diminuendo will require too much relaxation in the IAs which would result in breathiness. Yet some singers have very strong IAs and may be able to do a tension-filled crescendo and still manage a gradual diminuendo. That ability may be exciting but it is not an ideal form of a crescendo-diminuendo. Appropriate strength should make it possible for us to make sound in ways that appear effortless. That is to say, the correct muscles exert such that the phonation process remains flexible and fluid. Strength that creates rigidity or extreme tension in the singing process is ultimately harmful to the long-term health of the instrument.
What about the breath?
The kind of dynamic regularity that is necessary for a truly fluid and balanced crescendo-diminuendo can only be achieved if the breath is reactive. If I decide to take a cup of coffee and lift it, a number of muscles in my arm and hand response in perfect concert to achieve a move that is in fact extremely complex. Yet we make that very difficult movement look simple every day. That is because we take for granted that our brain is better than we are at calculating the precise muscular coordination for that movement. Do you remember when you were a baby trying to grab an object with your fingers and the extraordinary concentration that was necessary to do what now looks so simple? How many attempts over how many months did we invest before that move became fluid and not awkward and baby-like? How many muscles had to be balanced to accomplish what now seems like a simple task?
For a gradual crescendo-diminuendo, beside the complex muscular coordination discussed in the previous section, there is also an even more complex process relative to the muscles of breathing. We often think we have direct control of breathing and support because we can move the muscles of the stomach and ribcage at will. But we rarely stop to ask ourselves whether the movements we are able to access directly are precisely the ones needed for the vocal emission we are attempting.
The gentle, clean onset that is both clear and fluid requires very little air pressure, an almost passive action from the standpoint of conscious muscular control. Beginning the tone cleanly in a way that feels relatively "unsupported" is in fact the correct support for that quiet onset. From there, a crescendo calls upon the necessary muscles more and more until it feels that the entire body is involved in the crescendo. Then we attempt to maintain that coordination as much as possible during the diminuendo to avoid a sudden disengagement of the complex support system.
This "observing" the instrument at work and instructing it only by having a clear idea of what is to be achieved (like lifting a cup) is the ultimate goal of singing coordination. The mind imagines, the body does as the mind observes and does not interfere. Interference is simply activating extra muscular activity to help in a situation whereby we feel uncertain as to whether we can achieve what we set out to accomplish.
Some compensation is always necessary during early training. Just as a parent holds a baby's hands when they are first learning to walk, some muscular compensation will usually occur before the singer is strong enough to accomplish the job with the right muscular coordination. But just as a parent stops helping the baby and encourages him/her to walk by himself (even if he falls after a step or two), so must we let go of compensatory muscles and allow the natural process to take over as those muscles become strong and coordinated.
As always, in pedagogy, success of a particular technique depends on when it is introduced. Crescendo-diminuendo and other exercises such as coloratura-training and trills are advanced exercises that should be introduced when breath and fundamental phonation coordination has been learned and consciously understood. That a student makes a very good sound has no bearing on whether they know what it is they are doing. While basic phonation may be learned from childhood and properly influenced by the environment that the singer grows up in, it in no way implies conscious singing and does not help the singer go further when it comes to fine-motor-control tasks like crescendo-diminuendo and trills. Knowing how the instrument should work (i.e. what is our conscious part and what other parts are automatic) leads to high level skills. Some believe that knowledge is tantamount to interference. Singers who sing mindlessly will tend to be out of the way and for beginners that may not be such a bad thing. But when fine motor skills are necessary, they will eventually be lost. Singers who sing consciously can identify good tensions that come from necessary good coordination and the interfering kind that is "extra" and unnecessary.
Letting go is not mindlessness. It is a conscious decision to allow the instrument to work the way it is meant to because one becomes aware of the difference between consciously taking a good breath that uses only the necessary muscles and therefore feels free of compensatory muscular actions and one that is mindless and lazy. One is organized the other is not.
It is a very different thing to consciously decide to jump off of a cliff into the water below than it is to fall off of a cliff. The former is a calculated risk, the latter is mindless and dangerous.