A mood problem is being treated as purely mental, or a neck problem as purely mechanical — it's the coupling.

The mechanical link between mind and state. Rotation of the atlas (C1) and tipping of the axis (C2) twists the vertebral arteries — "as in a plastic garden hose; if you put a bend in it, you reduce or shut off the flow of water" — starving the brainstem where the five social-engagement nerves originate, which drops the body out of the ventral vagal state. In his classroom demo, one disturbing thought visibly rotated his C1 under students' thumbs, changed his breathing, and drained color from his face. And it cuts both ways: "Social engagement is not a fixed state" — the vertebrae shift the instant your psychological state shifts, quick to upset and slow to settle.