Just read this on https://www.youtube.com/watch?v=1BSJ-_4LKvE (Z Review - SMSL M6)
"Just found out that you can long press the knob to the right to toggle the Volume Control option (when it's ON you can control RCA's output volume, so it becomes pre-out).
Long press to the left controls screensaver (on/off).
Kind of annoying this is not even mentioned in the manual."