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The asymmetric separation on a rotating ball is due to the effect of the rotation of the ball on the boundary layer, called the Magnus Effect. The boundary layer is the very thin layer of air flowing around an object that "sticks" to the object. When a rotating ball moves through the air, the rotation affects the separation point, which is the point where the boundary layer stops "sticking" to the ball. On one side, it "sticks" a bit longer while on the other side, it stops "sticking" sooner. This causes the air to "deflect" in one direction slightly, which pushes the ball slightly in one direction.