The brake (parasympathetic system)
The half of your nervous system that slows things down. Long exhales and cold water switch it on.
Your body has an accelerator and a brake. The accelerator, the sympathetic system, gets you ready to fight or run. The brake, the parasympathetic system, brings the heart rate down, restarts digestion and lets you rest. Most of the brake runs through one long nerve, the vagus, from the brainstem down through the chest and gut. Evidence: solid
You can't decide to press the brake. You can do things that press it for you. Breathing out slowly and cold water on the face are the best shown. Evidence: decent Humming and being with someone you trust are barely studied but point the same way. Evidence: worth trying
Why it matters
"Calm down" is not an instruction the body can follow. "Breathe out for six seconds" is. Most of the tools on this site that work fast work through the brake. Evidence: decent