You flush the media to rid it of excess salts etc or to adjust the ph counterbalancing it, high ph soil flyshed wuth lower ph water to bring the ph to where you want it slowly over time. Or flushing the plant of excess salts amd nutrients by feeding it only plain or ph’d water in the belief that the plant will use up nutrients it has stored in its leaves and vegetation to produce a cleaner end product. There are pages and pages of people arguing whether it produces a better smoke or not, but from what ive learned here, people like to feed til the end not depriving the plant of anything it needs in its final days, and that just makes way more sense to me. I have never noticed a difference either way in the quality of smoke, but have also never done a side by side to really investigate the issue!