The first flush, for those of you who don't know, is a larger diameter pipe that is attached to the side of the house vertically, is closed at the bottom, with a valve that can be opened, and has a holding capacity of about 5 gallons. There is also a small ball in there which floats on top of the water, and plugs access once its completely full, diverting water to the diagonal pipe which flows to the tanks, for the rest of the storm. The purpose of this is to allow the roof rinse water- that first 5 gallons that flows over roof that's long been dry and has potentially collected dead animals and poop on it- to be diverted, and not stored in the tanks along with all the "cleaner" water.
Thanks so much to everybody who was here helping install the tanks earlier this year. Last I checked, this morning, it appears that the first flush is full and the water is going to the tanks now. Bring on the rain!