What is the purpose of rolling the ancestor rock down the hill? Chapter 18

When they reach the top of the hill, Edwin tells Cole to give the rock a shove, letting it roll back to the bottom. He tells Cole to imagine his anger in that rock, and that his anger is leaving as the rock rolls away.

Edwin smiled.“There’s a better way. Once the rock is set down, it changes meaning. Now it becomes your anger. Go ahead, roll the rock down the hill. Roll away your anger.”