I have not found complete documentation on the exact operation of each pin and the signaling between a camera (say a T6i) and a flash (say the speedlite 600ex-rt ii). What voltage levels, what is sent from the camera to the flash and vice versa, communication sequence, and so on.
I've at least collected this short-circuit trigger to ground that triggers the flash (I guess at maximum and / or some settings and immediately?).
The best image I've found until here comes from here.