Continuous feeding then flush, flush what

Im reading that you continuously feed for 5-7 days and then you flush your system. Does this mean that you drain your system completely, the reservoir and your containers? Or do you only flush the reservoir? If you only try and flush the reservoir wont all the containers try and flood the reservoir?

Just a thought not a sermon.

You just replace your nutrient water with plain water

Earl answered my flush questions in this thread.

https://www.rollitup.org/hydroponics-aeroponics/106454-final-flush-clearx-plain-water.html

I wasnt talking about the "final" flush. I was talking about the flush you do throughout your grow. If you feed for a week do you replace or flush your res and hold the straight water in for a week and then go back to the nutes? Is this to just keep everything safe? And is this something you kinda just determine when to do based on how the plants look and the level of your res?