The longitudinal magnetic field effect is caused by the characteristic flux motion driven by force-free torque. In this lecture, the peculiarity of this flux motion is discussed in association with the electric field induced. The reason why the variation principle cannot be used to derive the torque balance is also discussed. It is shown to be difficult to assume the flux cutting event as an explanation for various experimental results. Comparisons are performed relating to analogies in discovery between the theory of relativity and force-free torque from various aspects. Finally, the possibilities of superconducting power cables, the first application of the longitudinal magnetic-field effect, are introduced to show the enormous technological potential of this effect.