I saw this edge a few times when I was choosing edges. IT seemed like from the helpfile that I'd have to be out of the shadows a lot for it to be effective? Can you describe somewhat more how it works?

No stalk and no martial trance, and not a matter of how much you're hidden/visible.

That is to say, you see the increase in damage when you don't stalk someone or use martial trance. I could be dead wrong about martial trance, but I'm almost certain that "subterfuge and stealth" refer to stalking. So I guess that means you intend to fight someone, as opposed to stalking and attempting to assassinate.