The face feels ironed.
It's a curious statement to make, isn't it? A famous vocal pedagogue said these exact words it to me. They came to mind recently as I reread Paul Madaule's excellent book When Listening Comes Alive. He reminds the reader that listening is - in part- a vestibular activity, one that is felt. Speaking of which: snakes don't have outer ears, but they do listen. How? By sensing sounds via bone conduction (see here), a vestibular activity. Quite literally, they have a feel for sound. Singers do the same thing, of course, even if they aren't aware of how bone conduction works, though they are keenly aware, usually, when things don't feel right. The important thing to know is that it is just this aspect that is heightened during singing. In fact, it makes singing possible.
Hearing and feeling are two sides of the same coin.
So said the same famous pedagogue. This is a deft way of describing the two avenues for listening to tone, that is, through air conduction and bone conduction. To those who insist that the singer should not listen to what they are doing, I say: you are partially correct. That is, if you only listen to air-conduction you are indeed doing yourself a great disservice. Do this vigorously and you are yelling. Only a fool stops their ears and thinks they can sing! But oh...wait a minute. If you stop your ears and sing, you give yourself a perfect example of bone conduction. Try it on a slack /a/. Then on a vibrant /i/. What's the difference?
I submit to you that a vibrant /i/ feels and sounds different than a slack /a/. Is it so surprising then, since the facial muscles are neurologically connected to the inner ear via the Stapedius muscle, that their innervation should have an affect on vocal tone and be reflected in empiric teachings as evidenced in the quote above? As another pedagogue succinctly said: You will never get a bright tone with a dull face.
Older pedagogues insisted - despite scientific evident that the nasal passages were not resonators - that the student listen to the tone at level of the eyes. This is nothing more than listening to bone conduction, what I call the 'buzz' of the sound, which is not to be confused with nasal singing, and is readily accessed via /i/. If anything, experience has shown me that singers do not listen to this 'buzz' enough. It takes some practice to listen to heightened bone and air conduction together. A good image for this is a bow and arrow; the backward pull on the bow being the 'buzz' and the forward pointing arrow vowel clarity. The latter always seems to be 'out there'. Of course, everyone has their way of describing this, and I had to laugh when a young student recently said it sounded like 'WiFi.' God love him, I thought. He's right. There is no WiFi without a modem inside your head. Click on the 'buzz' network and go.