How does the VIOFO hardwire kit communicate that the car is parked?

All HK- cables do not communicate anything (in a smart way), all the magic happens in camera.

The HK- is simply a step-down voltage converter(s) with a low voltage shutdown function.
It does two things: it converts the ±12V/24V to the 5.2V required by the camera, and it cuts off the power at the selected low battery voltage setting.
It has 3 wires IN (yellow, red and black) and 3 wires OUT (yellow, red and black)
Yellow is ACC voltage, Red is constant voltage, and Black is ground.
The camera's USB-C port uses 3 pins to receive Y, R and B.

Both Yellow and Red must have voltage to turn ON the camera, without Yellow the camera with the HK- cable will not start.
As long as Yellow and Red have voltage, the camera will continue recording as normal.
Once the Yellow ACC voltage is OFF and the Red voltage stays ON, the camera will switch to parking mode and will return to normal mode if the ACC is turned ON again.
(Not that there is almost no current going thru Yellow wire).

Again, all the magic happens inside the camera (power controller/processor, etc), not in the HK-.
In fact, for My DIY LTO battery I am using 2 buck converters to replace HK- voltage regulator, and when using power stations to power the camera, I use 5V directly from the USB-A port, so I don't need the HK- voltage regulator, and I remove it.
See the link:

 
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The conclusion i came to was the dash cam monitors this pin, when it's high it knows acc is there and when low the acc is off/not available.

Are you agreeing with me and if so consider mentioning that in your reply otherwise it adds confusion.

Both Yellow and Red must have voltage to turn ON the camera, without Yellow the camera with the HK- cable will not start.

Good point about both Acc (Yellow Wire) and +V (Red Wire) needing power for the hardwire kit to start.

Do you happen to know which pin in the USB-C is being monitored and if it has any resistors etc in the it?
 
By @LateralNW : The conclusion i came to was the dash cam monitors this pin, when it's high it knows acc is there and when low the acc is off/not available.
Are you agreeing with me and if so consider mentioning that in your reply otherwise it adds confusion.
I do agree with you @LateralNW

I used the following USB-C Cable with Viofo HK cable and also with generic 3-wire cable, Color coding is axectly the same, Black - ground, Red - V+ power, and Yellow is signal - CC1=ACC, connected Black to Black, Red to Red and Yellow to Yellow, it works as intended:
https://www.amazon.com/gp/B0B24WVRCJ

Attached image is from Amazons product description and standard USB-C pin out (collar coding is the same except ground is shown green), you can connect tow images for Viofo's HK pin out.
 

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  • USB-C Cable for Viofo Mini2.webp
    USB-C Cable for Viofo Mini2.webp
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  • Standard USB-C pinout.webp
    Standard USB-C pinout.webp
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Here are couple of video links where I discuss use of HK-4 cable, removal of its voltage regulator and use of pair buck converters instead and use of USB-C cable.


 
In the image two posts up from GPak, the yellow ACC trigger wire (Viofo HK regulator) is connected to the Blue wire/CC2 pin on the UBS-C connector. I cut up a USB-C cable to do the mapping a few months ago.
 
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