If you have read Dr. Alfred A. Tomatis' book The Ear and the Voice, you've been introduced to a radical concept: the muscles of the ear integrate the muscles of the body with regard to extension and flexion. Of the two, flexion is more easily understood, and is what singers and voice teachers usually mean to when they talk about "support," which is felt as a condensing or gathering within the body—of muscles contracting. This is counterbalanced by sensations of "uplift," buoyancy" and "floating," which are signifiers of extension.
Extension is harder to train. It's what ballet dancers work on for the length of their careers. In that respect, bel canto singing is not-unlike being en pointe.
Perfect ease is experienced when extension leads flexion. "Forcing" happens when flexion overpowers extension, or when extension is absent.
My first instruction in extension came from Margaret Harshaw, who told me that the vertebrae of my head and neck should separate from each other. Years later, I experienced her meaning in a visceral way when I underwent Tomatis' Listening Training in Toronto, and felt an intense ache in my head and neck as my muscles lengthened after listening to filtered Mozart for a few days. Think sound has no affect on the body? Think again!
Having worked with flexion and extension in the voice studio, I observe that students are able to access flexion by the simple act of "calling," while extension is experienced through use of the breath: specifically by inhaling through the nose for 18 seconds—the very same procedure Francesco Lamperti taught his pupils. It is only when the student is well past "10" that the spine is felt to extend both up and down, and the rib cage fully open.
Extension is embodied in the Old Italian School phrase: inhalare la voce.