Philosophy
Where voice pedagogy usually fails is when there is a lack of objectivity and, therefore, clear tools to be used to produce a the desired outcome in consistently. Whereas it is possible to subjectively evaluate many elements of artistic performance, the path to competent technical execution is, in my opinion, best served by a clear-headed approach.
It is quite possible to be objective when speaking about intention, sound and related sensations. With the top 5% of talents, one can use a few simple ideas and they will sound just great ("talent" in singing usually being an alignment of innate kinesthetic and aural awareness, and very often a high degree of athleticism). With the other 95% of us, I believe it is highly necessary to get to the source of how and why things are happening, not with the goal of creating some innovative new technique, but to reconcile the old traditional ideas with what we now understand about acoustics, anatomy and psychology. In order to be professionally credible, we must define exactly what we mean by commonly repeated concepts like "resonance", "support" and so forth. Too often teachers will be slippery with definitions, explanations and justifications, to the point where nothing has truly been defined and the student is left to believe that there is some esoteric knowledge they simply aren't privy too or, worse, they simply don't have the "talent" to do it, despite many years of dedicated study. To be clear, not everyone is destined to be a professional singer or a great technician, but almost every willing person is capable of significantly improving their vocalism and understanding both intellectually and physically the most important technical concepts. Psychology is also an important aspect of technique. Your physical setup could be perfect, but it will collapse in about half a second if your mental intention does not support the kind of singing you hope to do.
On the subject of "old school" technical ideas: most of us are extremely far removed from people 200, 100 or even 50 years ago with our physical and vocal habits, and even the way we conceive of an operatic sound aurally (this last point I think has much to do with listening to far more recorded singing than live, accoustic singing). This means that we today usually have a very different starting point to those singers of the "old school" and, more often than not, there is be some remedial "voice building" required, along with correcting common postural and breathing habits, before the traditional ideas about feeling mask/palate resonance, appoggio and so on make any sense at all to many people. These are true sensations associated with high level vocalism, but it will not be possible to achieve the result by simply going for the feeling, as many voice teachers instruct. To be clear, there is no resonance truly occuring in the mask or even through top of the soft palate. However, when the vocal tract is properly aligned to enhance certain resonances, in a good majority of cases sympathetic vibrations will be felt in these locations and this is often important feedback for the singer.
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If we follow an objective path, a predictable and desirable result is the likely outcome. If the path is based upon a method of slippery, shifting definitions with faulty internal logic and subjective feedback, the result will be random. Perhaps, once in a while, it will even work extremely well, but you will come back the next day and be unable to produce the same result. As the proverb states: even a stopped clock is right twice a day. Technique should be freeing; a singer must ideally put their attention in performance on artistic and practical stagecraft concerns. The less objective and consistent a technique, the more a singer will be a enslaved by it. Finally, beware the schools which lean heavily on claims of objectivity and scientific veracity but which do not produce the advertised results. Those are the worst of all.