I have no idea really, but was watching and reading about flushing and found some things that could happen if not properly flushed and all that. In fact one of the things I saw about very dark burnt ash, I experienced recently with some dispensary flower. I was hitting it and it burnt super dark, smoky and stunk a little even thru a water pipe.
So that made me wonder, can you double flush to make sure nothing is in it or is 1 enough?
Actually properly flushed smoke 
the ash will be gray ur right on the money 
dark ash is improperly flushed bud but u don’t have to worry about that with the grobo 
Yep, the only thing you have to do with the grobo is wait until the Grobo switch to Flush mode (or force it)
Do a drain and fill, this time Grobo will not dispense any nutes and you root will be in pure water.
Your plant will probably need a little refill every 2-3 days
After 10 days your plant is flushed, normally it will have drained everything it could from the leaves…
OK good thanks.