This position of the back can be learned though hatha yoga. If you do not practice yoga, it is best to learn this through an experienced yoga teacher. I don't think it can really be learned through a book or a DVD. A good teacher should be able to tell you how you should be feeling through each movement and position. He or she should give physical guidance and correction to bring you into it. For most people who are not used to it, it takes time. I have a number of cyclists who come to my classes to get that freedom in the lower back and hips.