The Adem are called the silent folk, and they speak only rarely.
The old man knew many stories of the Adem. He'd heard that they possessed a secret craft called the Lethani. This let them wear their quiet like an armor that would turn a blade or stop an arrow in the air. This is why they seldom spoke. They saved their words, keeping them inside like coals in the belly of a furnace.
Those hoarded words filled them with so much restless energy that they could never be completely still, which is why they were always twitching and fidgeting about. Then when they fought, they used their secret craft to burn those words like fuel inside themselves. This made them strong as bears and fast as snakes.
Excerpt from The Wise Man's Fear
by Patrick Rothfuss