long answer how:
When driving in day time, the contact arm of the change-over switch 6 is set to the zero-position, which is designated by 9. The circuit will thus be entirely unloaded, and no current flows through the coil 3. The magnetic resistance of the core 2 will then be practically zero. This ensures that practically no magnetic flux passes through the core 4, when the coil 3 is unloaded. Practically the whole magnetic flux from the pole shoes follows the path of least resistance and passes through the core 2, which thus shunts the core 4. Thus, when the changeover switch 6 is set to oil, that is to say, when the searchlight is put out, practically no current will'be induced in the coil 5. If the coil 3 is now loaded by the search-light 7 being switched on through the change-over switch 6, the flux distribution will at once be altered because the core 2 then has a greater magnetic resistance, so that a more substantial portion of the flux will pass through the core '4. Thereby current will immediately be induced in the coil 5 and the light 8 receives current and lights up. Both the search-light and the warning light will thus be lit and put out by the same change-over switch, in spite of the fact that the circuit for the rear light is not electrically connected in the circuit of the search-light, in which the change-over switch is connected.
long why?
As a consequence, the previous drawbacks relative to the risk of a break down of the warning light, in case the searchlight should'be put out, are entirely eliminated through the invention.
short answer
identical to the tail and speedo light on the pug 102, except internal to the stator.
short why?
to prevent the brake bulb blowing when the headlight blows.