Im on day 17, but I will give this a shot. It is not at set up but occurs as you move your body and eventually the club into the backswing sequence. You do have a rounded back at set up and the only way it straightens is by lifting the ribs. So at the top of the backswing your head (if you drew a line in a slo mo video) would be a couple inches or so lower than it was at set up and also the angle of your face (lets use just the nose as a reference point) would be slightly up from where it was positioned at setup (pointing slightly more towards the horizon then the ground) Again, you are not independently lifting the head, your upper torso is lifting, like when you shoot a jumper in basketball.
Hope this helps, and I trust Tathata will chime in soon
Great question and great response. At setup we do want the back rounded and your upper body bent over the ball. As you move in the backswing, notice that you can push your glutes back (even 3-4” to feel it) and lift your ribs (chest up) and keep your head in relatively the same place. Your head does not have to lift up and away from the ball as you do this, your spine simply goes from rounded to flat. This is key to building and storing energy in the rib cage and upper body in the backswing. Do you still see how you could stay bent over the ball in the upper body and head remain in relatively the same place as you slightly lift your ribs in the backswing? Pushing back, as well as down in the glutes/legs should also help this counter balance. As mentioned, a good way to think about this is simply preparing to shoot a basketball and noticing what happens as the ribs lift and glutes push back as you prepare to shoot the ball. Great conversation, let us know if we can help clarify this any further.