Torsional stiffness is achieved by the way the ski is constructed. I remember having a pair of skinny skis about 20 years ago with low-density foam cores. I could twist them in my hands. I chattered like I was side slipping down a wash board road. I don't think anyone makes tele skis with low density injected foam today.
Most skis achieve varying amounts of torsional stiffness by torsion box construction.
This concept is simply wrapping fiberglass soaked in resin taughtly around the core of the ski. The weave of the fiberglass varies and there may be carbon fibers woven in too. They may have more than one layer of this wrap and they may align the weave at different angles to the ski's core as they wrap it. When it sets up it becomes a rigid sleeve around the core. The fiberglass wraps also will affect the longitudinal stiffness.