[eyko]

It's spinning, and the amount of curve matches the amount of speed. The unpredictability comes from the fact that the axis is also spinning, but very slowly. It's hard to get the topspin and rotation on the ball, and still hit the target. But these guys train shots for a living.

[dmoo]

If you have a few minutes have a look at Juninho, one of the best ever. https://www.youtube.com/watch?v=NesfdRNtwsE

Also don't forget the boots are designed to work the ball, for example http://en.wikipedia.org/wiki/Adidas_Predator