1. Center the Weight
The sternum strap secures the shoulder straps to prevent side-to-side swaying, keeping the backpack's center of gravity aligned with your body's centerline.
2. Pull It Close
Tightening the sternum strap pulls the shoulder straps inward, making the backpack fit closer against your back and reducing backward drag.
Summary:
The sternum strap helps keep the weight centered and close to your back, enhancing carrying stability and comfort. Conserving your energy.