The reason for the "curve force" on a spinning ball is explained in Figure 14.21 on page 274. The streamlines around a spinning ball are closer on the side where the surface is spinning toward the forward direction (because the speed of the air with respect to the surface is increased). The pressure difference (explained by Bernoulli's principle) on opposite sides of the ball causes a force, making the ball veer. A tennis ball is about the same size as a baseball, but its lower mass means that the "curve force" has more of an effect. Throwing a curve with a styrofoam ball is even easier.
Work Step by Step
This is discussed on page 274.