Imagine looking at the same diagram from behind. The forward speed plus the spinning speed in the top area, the white area of the diagram, creates more lift than the spinning speed minus the forward speed in the bottom area, the gray area of the diagram.

The end result on the boomerang is a kind of twisting force, or torque. The top area of the spinning boomerang is being tugged by the lift more than the bottom area of the spinning boomerang.

This is important because of the physics of how spinning objects react to twisting forces.
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Because it’s spinning the boomerang acts like a gyroscope or a spinning top. When you throw a boomerang, flicking or snapping your wrist like you’d snap a whip, you apply a torque, a force that makes the boomerang spin on its axis. For our imaginary top, that spinning torque is A in the diagram. 
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Now suppose we apply pressure, (another torque) at the upper end of the top—that is torque B. The result is that the top tilts in the direction C. If I keep applying pressure, B, the upper end of the spinning top will make a small, circular motion that is a combination of the spinning force, A and the applied force, B. This resulting circular motion is called precession. A little pressure makes the upper end of the top move in a small circle, a lot of pressure makes it move in a bigger circle.
Secrets of the Boomerang

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