I got most of my intial background details from the manuals for the legacy series, then the Omnipressor plugin.  Another source is the reissued 2830AU rack hardware manual.  Most (but not all) of the information applies.

Yes, you will need a solid, general understanding of how compressors work, especially the interactive controls Attack, Release, Ratio,.Threshold.  Once you’ve grasped ‘normal’ compression behavior, it’s much easier to fold in the unique features that Omnipressor brings to the table.

For starters, sidechaining towards the reverb side will increase the random behavior of the compressor in the H9 / H90.  You’re samplng more of the reverb / less of the input signal as a sidechain source.