The spine twist looks pretty simple, but there is more to it than meets the eye. First of all, it isn't simple to sit on the floor with your legs outstretched if you have tight hamstrings. The pelvis will also often dump backwards too, so it takes length and strength in the back muscles to sit up straight. The rotation is also supposed to come from the thoracic spine. So keeping the pelvis from rotating (you'll know if one leg gets longer than the other) while you perform this exercise is challenging. Then finally the stretching of the arms from fingertip to finger tip gets those shoulders. It feels great.

As I mentioned to Katie, don't be worried about using props to help your body perform the exercises better.