Power is so characteristically calm, that calmness in itself has the aspect of strength.

Agitation broadcasts need, and need is what people read as weakness. Real clout rarely has to raise its voice, so calm has come to look like capability whether or not any capability sits behind it. That is useful to know from both sides of the exchange.

In your own dealings, slowing your speech and leaving a pause before you answer changes how the words land, and it buys you time to think. Reading others, remember that composure is an appearance anyone can practise. Ask what a calm person is actually able to do, rather than letting manner answer the question for you.