Rocking requires length through the front of the body. Most of us are folded in half a large percentage of our day. We sit with our hips bend and our spine slouching. So this exercise is particularly hard for our modern lifestyle. It requires opening the hips, stretching the quads and bringing the spine into an extended position. If you can't get movement in it, don't worry. As you can see with Katie, that is a bonus. If you can lay on your belly and reach your feet- that alone is an accomplishment!