The sw triggers the pre-resistor relay circuit,which feeds R15, which drops the voltage to the fan motors ,resulting in low speed....... BUT...if BOTH the sw for low speed AND the coolant temp high speed are called for at the same time, high fan wins...........all that the high pressure sw/relay do for the fan feed is they add the R15 resistor to the circuit..if you were to jumper the R15 while low fan was running, you would wind up with high fan.
The sole purpose of the R15 resistor is to limit [ drop] the battery voltage to the fan motors..