Fishing wire attached to the board in front of the gun pulls and trips the mousetrap which then pulls the trigger. I guess keeping the right amount of tension on the fishing line is the important part.
The board in ‘front’ of the gun is non-moveable. Why show a picture of a gopher trap and leave out the essential component (the fishing wire you speak of) ?
Why wouldn’t the BAIT be on the ‘board in front of the gun’, going by your theory that a fishing wire would be attached to the front of the board. Are gophers attracted to bare 2x4’s instead of food ?