OK
Here is a ruff drawing of what is happening
The Temp probe is placed on anything you are trying to check temp at. Return line, Rad, Cpu block. Remember that the unit is only sensing temp NOT giving you a LCD or LED readout. The probe is a referance point for the board.
Pot #1 is a variable pot, by increasing or decreasing the setting you can have the Fan come on at higher or lower temp settings. AGAIN you must use an additonal thermometer If you are trying to get it to turn on at a specific temp.
Pot #2 controls fan speed, also a variable pot. Once you have determined when you want the fans to come on you would adjust the Pot #2 so that the fan starts turning as soon as the Pot#1 reaches its predetermined temp setting. Increasing the Pot #2 Will make the fan reach 12 volt output faster, Decreasing Pot #2 will make the fan start out at around 6 volts and as temp rises on Temp probe pot #2 will increase the voltage to the fan untill 12 volts is reached.
At the same time, as the point of temp probe cools the fan will slow down until it reaches the point of less than 6 volts where the fan will basically stop. ( takes about 6 volts to get the fan moving and it will stop turning when power drops to about 5.75 volts.
So in recap:
Being totally Variable you set the point at where the fan will start to come on and then set the fan speed as far as how fast you want it to climb to 12 volts.
Thus Totally automatic Variable Speed Fan control Circiut.
Hope this makes it a bit clearer.
Aardil