Dan, I have been fletching my own arrows for a while. From experience, the natural curl of the feather does not make the arrow spin because the feather is still aligned with the airflow. The air just flows along the feather. When you offset the feather, the airflow hits the side and pushes it away (around the shaft) and causes the arrow to spin. I have had to re-fletch some arrows because only one feather WASN'T offset, and my arrow just wasn't spinning right. All your feathers have to be either left or right wing. The feather spine is straight in relation to the arrow shaft, but as the feather comes up out of the spine, it curves over. Hope that helps.