Originally Posted by TboneZX3
Unlike other "open" (non-locking) differentials, in the MkIII Focus they would both spin, because of the Torque Vectoring system. If it were solely an open differential without TV, you'd be correct--only the wheel with the least amount of grip would spin.
^^This. The Torque Vectoring system would ensure that both wheels are spinning at the same rate in the case of sitting on ice. Because of this, if the TCS only gauged itself on difference in wheel movement it would never engage. This is one of the big reasons TCS systems have stayed away from that type of measurement. It's still part of the calculation, but only a small part.