What exactly is the relationship between Flow/Feed Rate and (Print) Speed?
By turning the knob in the main screen, you're adjusting "feed rate".
This is essentially a factor that all g-code speed settings are multiplied with - "speed dial" seems an appropriate name for it.

"Flow rate" is something different altogether - this is multiplied with the extrusion commands. It has the same effect as changing your extruders steps-per-mm.
You can adjust under- / overextrusion with this on the fly.