Flushing is the process of dumping gallons of pure water through the substrate the plants are growing in. The plant takes in that pure water and using a process call Flushosynthesis it removes all the leftover nutrients. Flushosynthesis should not be confused with Photosynthesis which is the process in which plants convert light energy, H2O, and CO2, into O2, glucose, and other sugars to feed the plant. The O2 is released into the atmosphere.
I'm surprised you have never heard of the process of Flushosynthesis.