It's over 10 years since I worked on my S2's headlamp motor circuitry so my memory is a bit fuzzy but this is what I remember.
The headlamp lift motor has a pinion on the end of it's driveshaft. This engages with a "crownwheel" like in a differential. This "crownwheel" has the pivot and lever connected to it to move the lamp. On the back of it is a cam and in the housing are two sets of contact points, one for raising the lamp, the other for lowering it. When the lamp is fully down, the raise the lamp set of points is closed under spring pressure of the contact and the lower the lamp set is opened by the cam. Likewise, when the lamp is fully raised the cam opens the rasie the lamp contact and the lower the lamp is allowed to close.
The changeover relay switches the 12V feed between the contacts. The relay has a NC and NO output, i.e. it switches the 12V output between the two when energised. Thus, with the lamps down and switched off, the raise the lamp side of the relay is open and the lower the lamp contact has 12V. The motor does not move because the cam has opened the contact in the lift motor. Turning the lights on energises the other side of the changeover relay and, as the raise the lamp contact in the motor is closed, the motor runs to raise the lamps until its contact is opened by the cam. Turning off the lights puts 12V to the other output (NC) of the changeover relay so the motor runs to lower the lamp until the cam opens the lower the lamp contact in the motor.
There are diodes to prevent the supply back feeding from one to the other side of the motor. Sometimes a contact in the motor welds itself shut so the headlamp cycles up and down like a mad thing. Sometimes they get dirty so the lamp won't go up or it won't go down depending upon which contact is affected.
I hope that helps.