These use more than just a fast idle, so on Transits they either use the dedicated speed control options, or piggy back the throttle pedal potentiometers to manipulate the pedal position.Ford 'fast idle' is implemented in two ways, by BMS signalling on private LIN to the BCM & then by HS CAN to the ECM to raise the idle speed due to low battery voltage or by the HVAC module signalling high aircon load, again using HS CAN to the ECM.
In a traditional (i.e. throttle cable implementation) you could use a vacuum solenoid to pull on an auxiliary cable attached to the cable linkage at the injection pump and use a pressure switch to control the solenoid - similar to how Ford implemented 'reverse fast idle speed' on Escort diesel vans in the 90's
The older transits used a resistor ladder setup (there was a wire loop in the harness you cut open, and could insert resistors to control speed in various ways, combined with setting RPMs via IDS), but I've never actually looked that far into the newer transits.
I can tell you that even Winton can't supply a wiring diagram for their systems.
How they get wired up, largely depends on who happens to wire them in on the day they get converted.
I've seen seemingly identical vans of similar age, where one has used the speed controller, and the other used the throttle pedal.
I've also had one where Winton cut into the clutch pedal switch, which prevented it from regen'ing, but Winton couldn't tell me why they done it, or how/when it cut the signal.
Out of all the OBP systems I've dealt with, Winton give the most problems.
They like to think they're an engineering company, but all they engineer are the mounting brackets.
They buy in most parts, and just bolt them together however they can get them to fit, with no thought given to servicing them.