Hi Alexandra
Each class you may feel a little differently in your body so keep attuned to that. A couple of things to try.
** The shoulders towards the hips really works well, see how you can enhance that. My recent video may give you some clues. A transformational technique for your practice
** Try bringing your knees apart. Play with different distances apart and notice that your chest rises and your back arches well in this way. As you get more flexible you will be able to bring your knees together and still feel your lower back surrendering somewhat.
You are likely always to feel some compression in your lower back in any case. See how your body geometry responds to different knee distances and the way you hold your arms.
Namaste
Gabrielle 🙂