The yogis believed that your life-force, prāṇa, travels on your breath, and that if you can steady the breath you can steady the mind that rides on it. They built a whole practice around the idea, prāṇāyāma, and spent centuries refining it.
You do not have to believe any of that to use it. Breathing is the one automatic system in the body you can also grab the wheel of whenever you like, and through it you can nudge the rest. Make the out-breath long and slow and your heart rate eases off, the body reads it as a sign that you are safe, and the tension starts to settle.
That is why a few deliberate breaths can reach a moment that nothing else can get to: stuck in traffic, waiting to walk on stage, halfway into an argument you wish you were not having. Nothing to buy, nothing to carry, always with you. The old teachers wrapped it in ceremony, but the thing doing the work underneath is plain physiology.
Try this: Breathe in while you count to four, out while you count to six. Do that six times. Reach for it right before anything that winds you up.