I can’t really speak about compression/mastering right now, both are very deep topics that a few paragraphs won’t do justice, and I’ve only just started to get an idea of how to use compression properly in the past year or so myself. Have a look at some online guides, they’ll provide better knowledge than I can!
The OP-Z compressor, though, is accessed using the yellow encoder while holding the mixer button. It’s a fairly basic application of a compressor, but essentially, any loud sounds will “duck” the audio so it doesn’t go over a certain level. For example, if your bass is too loud, it’ll cause everything else to duck. This can also happen with inaudible sounds, ie. if your bass goes below the lowest frequency your speakers can produce, it’ll still affect the compressor.
Having said all that though, I quickly played with a few patterns tonight, and noticed that turning the synth group up too high in the mixer page caused the sound to cut out… so I wonder if it’s got some kind of automatic temporary mute for when the sound gets too wild…?