Sustained tones. Manuel García started every lesson with them, which, experience teaches, are not for the faint of heart.

They are the equivalent of the now popular "plank" pose (more accurately called chaturanga), where the body is held in position for a long time so as to develop "core." You really have to decide that you are going to do both maneuvers with great intention or all bets are off. 

What must one have in order to perform sustained tones and the plank? Extension, which must rule the body's other great force—flexion. 

Telling this to students is one thing. Getting them to exhibit it is another. Standing up straight? Nice try, but that's not it. The baritone hunched over singing Rigoletto needs to exhibit extension just as much as the recitalist who stands still.

Breathing puts one in touch with it. However, it must be present even before a breath is taken, and is the product of an open ear, which is reflected in an open face, elongated spine, and open rib cage.

The old school called this the "singer's sensation."

One way to find it? 

Close your lips, separate your teeth like you are cradling an egg on your tongue, smile to yourself gently, inhale slowly into the middle of your head, and then suspend your breath for a good 5-10 seconds, all the while feeling the muscles of your head, face and body lift and expand.