Stand in the middle of an open doorway and use your arms to apply outward pressure on the two sides as if you were pushing them away from your body. (scientificamerican.com)
Instead, they are dynamic structures which must counter outward pressures to prevent their swelling or rupture. (sciencedaily.com)
Once you've mastered that, you will no longer need to put the inward or outward pressure because you will have the balance. (weckmethod.com)