It is as if all of the gravity forces on the atoms which make up the disk act as one gravity force, F G , through the center of mass. Even more important, an object tossed into the air will rotate about its center of mass. The plane also has a center of pressure. The disk behaves as if all the forces of air act as one force, F A , at the center of pressure. The center of pressure of the disk is at the center of the disk.
Gravity pulls down at the center of mass, the air pushes up at the center of pressure.
As the plane falls under gravity air pushes up on the wing acting like a single force at the center of pressure, the center of our disk. Gravity pulls down at the center of gravity. If the center of gravity and center of pressure are close together the disk tumbles through the air. However if the center of gravity is moved toward one edge of the disk by adding weights. There will be a twisting force, or torque, on the disk.