AC condensor fan speed. What controls it?

[2TDIguy]

So if I turn my key on with AC on the fan runs fast. Seems there are 2-3 speeds) My problem is the AC is kicking out in stopped traffic as it is likely hitting the high side cut out psi. I popped the hood and the fan is just idling along at that lowest speed. What tells the fan to run on hi speed?

 

[wtribeflyer]

High side pressures or high temps. I'm guessing.