For me, the "low and back" is in relation to the spine/torso. In the system I like and teach, you start with the shoulders (both of them) relaxed and down. That, to me, is the natural "down and back" position. Note the relation of the shoulders to the spine and then leave them in that relative position. They move in relation to the bow arm and bow but not in relation to the upper body/spine.
Now as you draw, and turn your upper body to align the shoulders to the target (from the open position) the shoulder is moved forward in relation to the bow arm but again, doesn't move in relation to the body. (Tried to say this twice, as I see it as an important distinction)
To "see" how this works try this experiment. Stand up straight and just relax the shoulders to their normal position. Now, place your hand on the collar bone (bow side). Now, if you move JUST the shoulder forward, can you feel the collar bone move or change its orientation to your body.
Next do it again but just turn your upper body to move the shoulder. See how the collar bone does not change its relative position? This is the way to make that shoulder alignment. It is not an independent movement of the shoulder but a result of turning the entire upper body.
So your shoulder starts out down and back in relation to the upper body and stays in that position. It doesn't "shrug" up or "shrug" forward by itself.
Again, one of those things easy to show and hard to say in words. But hope that helps.
Arne