pulled the trigger back one notch

He pulled it back just a little bit. There's no such thing, literally, as a 'notch' on a trigger. So the meaning is he pressed the trigger back by a fraction of a millimeter. One more fraction and the weapon would fire.

Well, there is such a thing as a set trigger using only one trigger, but I would bet you're right, Merry Andrew. Either that, or someone should have said "He pulled the hammer back on notch" (to the half cock position), which nobody with the sense God promised a lug nut would use.

Hi, Rog. Yeah, I thought of that. This might have been written by someone not really familiar with firearms or firearm terminology. To say he pulled the hammer back a notch would make some sense. You don't actually pull a trigger back. You squeeze it.