a) At the beginning of her first lesson Nicky's arms were protruding behind her body. This is a common picture of long armed riders. Her back was hollow and she was rising behind the horse's movement.
b) At the end of her third lesson Nicky is in a slightly forward position with her shoulders ahead of her hips. This position places her centre of gravity over her base of support and over the horse's centre of gravity. It allows her to move in balance with the horse's movement.