Up till now, she has only used the primary receiving position and falls forward to block. We are working to change that.
Even in the primary/ low receiving position, the throwing arm shouldn't be behind her back. It's unnecessary. My preference is to see the hand tucked down next to the ankle.
In addition to the other issues brought up in other comments, 1 arm behind the back during blocking makes it much harder for her to get her shoulders square to the path of the ball. Angled shoulders/chest results in blocked balls being deflected sideways instead of staying in front.