Components of the airfoil.
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http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/Jon_Drobnis/Curveball.html
The effect is apparently similar to how an airplane wing works, in that a difference of air pressure on one side vs another causes the object to move in a particular direction, the one of least resistance.
From Wiki: Airfoil
“The lift on an airfoil is primarily the result of its shape (in particular its camber) and its angle of attack. When either is positive, the resulting flowfield about the airfoil has a higher average velocity on the upper surface than on the lower surface. This velocity difference is necessarily accompanied by a pressure difference, via Bernoulli’s principle for incompressible inviscid flow, which in turn produces the lift force. The lift force can also be related directly to the average top/bottom velocity difference, without invoking the pressure, by using the concept of circulation and the Kutta-Joukowski theorem.”
http://en.wikipedia.org/wiki/Airfoil
http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/Jon_Drobnis/Curveball.html