The act of breathing is really just you expanding your ribcage with muscles and the diaphragm under your ribcage to make the chest cavity bigger to pull in air. Your arms are also attached to the ribcage with muscles, and usually the ribcage acts to support arm movement. But if you anchor your arms, you can do the opposite – using arm muscles to move the ribcage. This helps the chest cavity expand more so you can breathe better.
If you google ‘tripod position breathing’ there’s more detail if you need it but that’s the gist.
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