Unlike backpacks or messenger bags, BackTpack, with its balanced side bags, applies the load to the body’s vertical axis, thereby eliminating posture distortion and decreasing the effort required to carry the load, in obedience to the laws of physics. The load feels lighter and you stand straighter.
The Center of Gravity
When we support a weight, we have a new center of gravity -- the combination of our body's center of gravity plus the center of gravity of the weight we are supporting. When what we carry is balanced left-right or front-back, or is on top of the head, its center of gravity aligns vertically with ours.
However, if the weight is off-axis (in front, to one side, or behind), we must lean away from the load to keep our new center of gravity over our feet so that we don't fall. Posture must change to accommodate the bag we carry, and the heavier the load, the more we must lean.
Line of Force
The force applied to support the load must go through the center of gravity of the load. So again, when the load is balanced, or on our head, the supporting force is simply equal to the weight we carry, and in the opposite direction: straight up!
When the weight is off-axis, we must pull in a direction such that the direction of the pull goes through the center of gravity of the load. This introduces a horizontal component to the force, which is added to the weight, meaning that more force is required. Put another way, a heavy backpack requires you to lean forward and pull forward with your shoulders. This is why you see hunched shoulders and forward heads with backpack use. With the same load in a BackTpack, you eliminate the forward pull.
It is also the reason that people say, "The load feels lighter!" when they try on a BackTpack loaded with the contents of their conventional backpack.