The purpose of a NTS system is quite simply to prevent the propeller from driving the engine in the scenario where you have a low power setting and sufficent airspeed for the airflow to cause an increase in prop RPM. It serves to limit undesirable negative torque that would result in this situation (i.e. torque being absorbed in the opposite sense than that for which the system was designed). In certain aircraft the NTS system is an integral part of the airstart process, and you would need it to be servicable to restart an engine that you had previously shut down. Its action not only limits undesirable torque during an airstart, but helps prevent a massive overspeed resulting from a windmilling prop being driven to an overly fine pitch.