Stun Damage?

How does this work? It is confusing to me...

At first I thought it worked like sub-dual damage but then the official description did not make it sound like that, so I don't know.

Stun damage means you're stunned. That probably means doing anything too complex is defying danger, the danger being "you're stunned."

GM: "While you're trying to talk your way out of the bandits, you don't notice the one circling around behind you, who blackjacks you right in the skull. He's just trying to knock you out, so it's stun damage."
Player: "Gah, okay, I draw my sword and run him through."
GM: "Well, your head is spinning, and your arms are kind of heavy, you've got to defy danger to keep it together. On a 6- you'll probably pass out."

Thank you. That helped a lot.