Here is the schematic..you can see how the relay brings the R15 dropping resistor into the circuit to accomplish low fan . You can also see how the other relay [ coolant temp sensed] by-passes the resistor so the same fan motor gets full battery/system voltage, resulting in high fan operation.
The confusion on these fan circuits is the fact that there are two different circuits , but they both SHARE the same fan motor/motors...
This shows that design and makes it quite simple.

http://members.aol.com/ajdalton7/auxfan.jpg