The coil for the relay at the top gets hot from the ignition key

The air thermostat (11) measures temperature of the air behind the radiator, at a certain point, it closes the circuit, energising the coil in the relay at the top.

This relay allows power to flow to one fan (7)

the ground wire for fan (7) goes back to terminal four at relay (13) on the left wheelwell. This relay is a Single Pole Double Throw. Unenergised, it allows terminal three to source its power from terminal four

Terminal three is connected to the second fan (8). The second fan gets "used" power of the first fan. Since the two fans are connected in series, each gets 6 volts of power, and runs at "half-speed"

The relay coil at the top gets power from the ignition key, it is activated by radiator air temperature switch (11). This is the same as in low-speed operation.

Thermostat (10) measures the water temperature in the radiator. When it activates:

Relay (13) coil energises, so that terminal three gets power from terminal "S". Terminal three supplies power to fan (8) This power is straight 12V from the battery.

Relay (12) coil energises; this connects the ground wire for fan (7) directly to ground. As this ground wire has no other resistance in its circuit, the has a full voltage drop of 12V. Both fans receive 12V power from the battery, both fans are directly connected to ground, with no intermediary drops. Fans are connected in parallel.