This allows a standard PC to control an external fan with a brushless ESC.
The fan is being controlled from a standard PC fan controller card. This generates and output in the range 0-12V, which would normally we used to power a simple analogue fan (where lower voltage goes slower) within the computer.
However, our fan (taken from a Honda Accord) is driven by an external Turnigy Brushed ESC. This takes as an input a 50Hz PWD in servo format. In this case a write cycle of 1000us corresponds to zero RPM and 2000us corresponds to maximum RPM (estimated 2500 RPM for this particular fan).
We need to send a signal back to the PC fan controller card, to report on the RPM being used. This a frequency modulated (FM) signal - expecting two pulses per rotation of the fan (known as "tone"). However the lowest frequency that can be sent is 31Hz (corresponding to 31 / 2 * 60 = 930 RPM).
The PC keeps a rolling average of the frequency data it is sent. Thus to report 465 RPM we can alternate reporting zero and 930 RPM.