Set the forward lean of the rear highback.
The rear high back does the same thing as the front high back which you have already set. The two high back together allow you to control the edging on the heal side edge of the snowboard.
If, while standing two footed, hips facing the same direction as the front foot, you rock on your feet towards the heel edge of the board, you should feel both highbacks contact the back of both legs at the same time.
You should also be able to balance on the heel side edge using the high backs. Notice the rider balancing on the heel side edge while remaining in a relaxed stance.
It is the ability to have small movements such as these, that enable balanced, effortless riding.
Take it for a ride.