The pic was a little brainstorm creation between 2 guys discussing going on vacation. It doesn't change anything about the DWC setup, except that it maintains water level for you. Think of it as being like one of those 'auto-watering' pet feeders. I have done something similar with a swamp cooler.
I can pretty easily describe how it works for you...not much info in the thread where I found the pic. For one, the 'fill res' does not need to be that big, one gallon or so would be plenty. The fill res needs to be strong, like you can't use a plastic jug. Should be something non-collapseable like pvc, glass etc. Reason why...the fill res will rely on the vacuum at the top to keep the water from flooding the main res, so if it collapses, water will escape without air entering it. Basically, once the water level drops below your fill hose, air will enter the fill res, allowing water to escape until proper height of main res is reached and air supply to fill res is once again blocked.
Make sense? Sometimes I am not the best at describing things.
'Nother point...you need to have a valve to open and close on the fill hose, so the main res can be removed and refilled. This is an added bonus, great place to premix nutes
. My thinking is, when your girls get pretty big, you might find yourself 'topping off' your main res several times a day, to maintain optimal water height. This could be a pain, not to mention increase the chance for shock from slight variances in water temp or ph.
Let me know if you need more info, I'm sure I could locate a thread with a better description if necessary. Or perhaps one of the big hydro guys like Kade or Stoney will pop in to help
.
Good Luck brother