I wasn't aware that the Rheobus featured a channel with direct access to the +12v and +5v rail. That's cool. The controller that I have can only switch between +12v and +5v, and not voltages in between.
It doesn't. Just feeds off the +12v rail and disperses it among 4 channels. However, the rheobus is rated for up to 20w of power, most fan controllers I've seen say 15-18w, whereas like I said rheobus just says straight 20 not 18-20 where it might be 18 on one day, 20 on another.
Anyway, turning the knobs is just cutting the amount of voltage going to the fan, but they simply givin an 'indicator' if you will. You can actually click the knobs off too. 0-7v = Red LED, 8-12v = Blue LED.