This paper discusses the operating principle of a rotational energy harvester for body motion with an eccentric proof mass. A mathematical analysis for the rotor motion under different excitations is performed and the gravitational and inertial operation explained. The transducing mechanism works on the principle of frequency up-conversion that is now widely used to harvest low frequency vibration more efficiently, and uses a piezoelectric beam and magnetic coupling. A miniaturized device with an overall size similar to that of a wristwatch is introduced. The fabrication is entirely done using standard milling and turning processes. Experimental results for this device show significant improvement in the attachment of the piezoelectric beam compared to a previous prototype. Furthermore, there is a good match between the magnetic forces and the proof mass for the tested excitations. A disadvantage of the miniaturized prototype is the higher stiffness of the piezo beam, preventing free oscillation after actuation. Modifications to counteract this problem are provided and experimentally validated.