In Chap. 1 , the pole-changing effect of FMMs was explained as being achieved through the utilization of the modulation unit. However, the physical principle underlying the modulation unit’s influence on altering the pole number of the magnetic field requires further elucidation. Additionally, the specific magnetic field components generated under the flux modulation effect have not yet been determined. Consequently, this chapter will initially analyze the variation pattern of the magnetic field under a non-uniform air gap structure. Subsequently, the MMF and specific permeance model will be established. Building on this foundation, the flux modulation processes of different structures—namely, the surface-mounted permanent magnet structure, the doubly-salient structure, and the embedded permanent magnet structure—will be examined, culminating in the development of the fundamental theory of flux modulation.