The 3-hour magnetic activity index, Kp, is widely used for measuring the level of magnetospheric activity, and many magnetospheric properties are known to correlate with it. The common denominator for these different properties is the strength of the magnetospheric convection electric field, the large-scale electric field imposed across the magnetosphere by the flow of the magnetized solar wind past the Earth. While the relationship between Kp and the global convection field has long been known, the question of why the relationship exists has apparently not been addressed. In this report, it is proposed that because Kp is derived from magnetic variations obtained at subauroral stations, it is extremely sensitive to the latitudinal distance to the equatorial edge of the auroral region, where the principal causative currents flow. Since the auroral region maps to the plasma sheet in the magnetosphere, motion of the inner edge of the plasma sheet, which is determined by the strength of the convection field, causes significant changes in Kp. Thus, through its dependence on the latitude of the auroral current region, Kp can be viewed as a direct monitor of the strength of magnetospheric convection, explaining the success of previous Kp-dependent models of the global electric field.