I watched a tutorial that said you could add pins to a custom HDRI that would enable the light to be, sort of, sucked out of specific places by giving their brightness negative values but I have tried this and it doesn't seem to be doing anything, is there something I am doing wrong or is there a setting I have missed??
have you tried checking the "blend" box?
And made the brightness negative?
Yes done both, it was actually a bit strange, later on Keyshot crashed but before, I had saved the new HDRI, when I re-loaded the file it later showed the HDRI file WITH the negative values despite at the initial time it not displaying them.
Quite weird?