Refer to figure.
The three-phase alternate current from the engine-driven generators drives a synchronous motor. A permanent magnet rotating in an aluminum drag cup is linked through a shaft to the rotor of the synchronous motor. Eddy currents with their own magnetic field are generated by the rotating magnet and act on the electrons of the drag cup. The interaction between the magnetic fields produced by the Eddy currents and the magnet's magnetic field produces a turning force that will turn the drag cup in the permanent magnet's direction.

A small shaft connects the drag cup with a hairspring and a pointer calibrated to indicate the RPM on a dial. The frequency of the AC supplied to the synchronous motor is proportional to the engine’s RPM. Therefore, the higher the RPM, the higher the current supply to the motor and the torque resulting from the magnet rotating inside the drag-cup. The hairspring opposes the turning force to control the indication of the pointer.