That's the Rule, which is so simple as to be overlooked. What does it mean?

Well, for one thing, knowledge of the Rule reframes the singer's idea of "support," which, I have to say, is not a word you will hear from my lips—and that is because it commonly refers to the action and sensation of flexion rather than extension. 

Flexion is easy. But to extend—which is what ballet dancers do in a graceful and unified manner, is not easy for many—and I would even say—most—students of singing. 

But you don't know what the hell I am talking about, do you?

Ok. So let's make this simple. 

Extension is what you feel in your muscles when you shut your mouth and gently inhale through your nose for 14-18 seconds. That moment of suspension before you start to exhale? That's the feeling of extension. In fact, if your proprioception is fully-functioning, you'll discover the feeling of extension somewhere around the 9th or 10th second. 

Got that? Try it and you'll see what I mean. 

Oh yes, your ribs will open (some of you will notice that I am sure) and that's fine and dandy, but if you pay attention, you will observe that all the muscles of your body—inside and out—from pelvis to crown of head—will feel lifted and extended.

This feeling is what must be cultivated regardless of the amount of air in the lungs. The old school bel canto teachers understood this as singing on the breath, and even thought of the feeling as indicative of a column. They also called it The Singer's Sensation.

By-the-way, we're not talking about rocket science or mystic mumbo-jumbo, but observable phenomena—stuff that happens in nature.

So here's the deal. If you make an effort to "support" your voice by merely contracting your muscles, you are flexing rather than extending. And your voice needs and wants extension. Once you get a feel for extension, flexion will by-and-large take care of itself. However, flexion can never do the work of extension. Guess what you need when you sing up the scale? 

Extension leads flexion. 

That's the Rule.