Sorry but you are wrong. Acc is only relevant for manual or ea modes. Nothing to do with ttl. Below a screenshot from inon catalog explaining what it is.
View attachment 550136
http://www.deduiker.nl/pdf/INON D-2000 Advanced Manual.pdf
Above is the link to advanced manual, I don’t see how you come up with a special functionality.
As you see in the below picture, "normal" state is magnet in which is meant for the majority of the cams, as most have pre-flash.
View attachment 550137
The magnet is only required to turn off the ACC that by default is on. The ACC has to be left on anytime the camera sends a preflash ttl or not is irrelevant. If you set the magnet to turn off the ACC the pictures get dark as the full power is done at pre-flash. Hopefully this is not the case with the op
The most likely cause of lack of sync is note L RC mode needs to be set to off. Even -2 exposure compensation does nothing to the flash, you need the flash ev to set to -2 which is not what is happening here