If the motor is coupled to the drum below it, it is an ordinary eddy current coupling to the tacho shaft and pointer. For a beginners lesson in automobile metering, 1, the pointer needs some spring to return it to zero in the absence of a driving force. 2, the driving force is normally magnetic although some older style meters use the expansion/contraction of a wire getting hot. Going back to the magnetic drive, this could be provided by the attraction of a coil attached to the meter's pointer spindle acting on a fixed magnet or magnetic material or can be provided by rotating a magnet close to a disc that is attached to the spindle of the meters pointer. This system was the de-facto speedo drive for the last 60 years. This form of drive I think is being used for your tacho.
Frank