Explain Why Box Breathing Actually Calms You Down

You've been told box breathing works, usually by someone selling a course. This explains what's actually happening physiologically: the vagus nerve, the heart rate change, why the hold matters as much as the breath. Useful before you trust a technique enough to use it under pressure, or before you teach it to someone else.

Explain how box breathing (four seconds in, four seconds hold, four seconds out, four seconds hold) actually calms the body down. Cover what is happening to the vagus nerve and heart rate, why the hold matters as much as the breath, and how long it typically takes to feel a difference. Avoid vague wellness language: ground every claim in the physiology. Keep the explanation to around 200 words, plain English, no jargon left unexplained.