In my 'homework' practice, I played with the relationship between centering my lower ribs, and opening my chest, while in downward dog. I felt like I found a good space there, with a little less arch in my lower back than I normally have, and a little more lightness in my upper back - and that's where I really have to pay attention. Flexibility, proper alignment, and awareness do not come naturally for me in that area.
I *think* I finally made the correction on my own that my teacher makes for me in class. That's a sign of progress!
Playing with that position of the ribs seemed to create a space for me that allowed me to feel the difference between my normal, and the correct alignment. My teacher talks about the butterfly effect on the upper back, where the lower ends of the scapulae fan outward like the wings of a butterfly, and the spine stands alone, centered, like the body of a butterfly.
After feeling like I made that correction, I couldn't recreate it. But at least now I think I know what feeling I'm looking for, and I can keep trying!