Ease the low back by rolling over the arms. The arms provide a kind of massage for the contraction along the lower back and ribs. Your contracted back muscles will feel supported in a whole new way. I love this lesson. (Have a rolled towel nearby if you are not comfortable with the arms behind your low back.)
Tip: Note that the knees are out to the side in this lesson. Do not allow the legs to hang on the ligaments. Stretching is strenuously ill-advised. Hold the legs up so they don't pull on your hip sockets. Each and every person will have a different comfort zone for this position. Yours might be high up, someone else's more sideways. It does not matter, you are still doing the lesson.