Flushing means water your girls with clean water. If you are using tap water make sure you get the chlorine out of it before use, as chlorine will kill the beneficial microbial life in your soil. This may be done several ways, leaving it out 24-48 hrs putting airstones in the reservoir will help chlorine purging. Flush means you are taking the old nutes you put in the soil last feeding and getting rid of them. Doing so will aid in feedings because you will have a reduced risk of overfeeding and burning the plants. See
The truth about flushing. I'm a believer in flushing until the water runs clear from the drain water. I have read that you should flush over other week with up to 3x volume of the container. When watering if you do not water
enough this will lead to dry spots in the soil where the water has created channels, via the root and soil mixture. Ensure you put enough water through or these 'dry pockets' may lead to problems with root development.
All these things are pretty variable depending on
YOUR situation and environment. If you live more north or at a higher altitude you may be more or less, same with if you live in a desert. Sorry if this seems long and I'm sure some might agree or disagree with what I've said, not many people have duplicate grows. It changes just from site to site. Your ventilation may be different so you don't need to water as much yada yada. I hope the link is some help. Get luck to you sir! Let's have fun and grow together. :}