A three-phase AC power transmission, when balanced, requires zero neutral current. By adjusting the magnitude of one phase or the phase displacement of a phase, you can observe the resulting neutral current.
Three-phase power transmission depends on a 120° phase relationship between each of the phases, as well as relatively even current magnitudes in each phase. You can adjust the magnitude of one of the phases by ±20% in order to observe the resulting neutral current. The Demonstration also allows you to adjust the phase relationship of one phase to illustrate the resulting neutral current if a single phase had a different power factor.