This paper proposes a topology of special rotor-type permanent-magnet (PM) machine named rotor-PM flux-switching (RPM-FS) machine, which is evolved from stator-PM flux-switching (SPM-FS) topology and exhibits high power density, wide-range speed regulation, and good overload capability. The operation principle of RPM-FS machines is investigated based on the field modulation theory. It can be found that the torque is mainly contributed by the dominant harmonics in rotor-PM field and armature reaction field with the same order and rotating speed, i.e., 10, 14, and 34 pole pairs. In addition, a comprehensive comparison of modulation principles between the RPM-FS and SPM-FS machines is conducted from three perspectives, namely PM field, armature reaction field, and torque production mechanism. It is found that the modulation principle not only reveals the similarities of operation principle between two PM-FS machines, but also provides guidance for the combination of stator slots and rotor poles. Further, the electromagnetic torque produced by the dominant harmonics in two machines is verified by finite-element analysis and experiments.