From the manual: "The In parameter can be also perceived as a threshold parameter available in conventional compressors."

As I understand it the drive control sets the threshold, adjusts the ratio accordingly and also apply make up gain.  So I wonder how does the In and Out buttons relate to drive.  It seems to me that they are redunant but I am probably wrong but I don't know how.  Is the In button for cases where the input signal is so low that you have to drive the compressor extremly hard to get a compression effect?  Some kind of preamp effect to give crunchessor a sensible level to work with otherwise it would/could create extreme ratios?  It would be nice of you to shed some light on this.

"In" knob was indeed added as a remedy against very low-volume signals.  But beside that it can be used as an additional compression control - for example, at the given Drive setting you may get more compression by increasing the "In" gain.