Good question! The reason he draws two c curves is because he's considering the form of the ribcage ball and hip ball. The centerline acts as a crosscontour line. Structure will be taught in the next lesson, and there you'll learn about crosscontours (lines that you wrap around an object, as if you were putting a rubber band on it, to show it's orientation in space. Like longitude and latitude lines on a globe.) Both balls are rotated away from us to the left, therefore the crosscontours are bent the same direction. The ribcage has a more bent crosscontour line, since it's more rotated away from us. But why not an s-curve? Instead of thinking "s-curve equals twist", think that "a twist can cause an s-curve". Whether you end up with an s-curve or not depends on the angle. If we were to rotate the model to the right, so that the hip is turning away from us to the right, while the ribcage is still turning away from us to the left, we will have an s-curve centerline. Hope this answers you question :)