Yeah I've never heard of flushing. I've only done outdoor before I came to a green state though. So I had no way to flush, or reason for someone to bring it up to me I guess. But it doesn't sound like it would do much. Unless your talking about draining it out twice then reapplying some kind of nutrient, that might be effective. But from thinking about it right now having extra nutrients left in the dirt from the past building up would be helpful as long as it's not killing the plant. And if you let your plant drink all the water in its pot before watering again, isn't that already giving the plant "new water", and if your reapplying your nutrients every time through watering, isn't it already getting new nutes. And water alone wouldn't give it nutes. Water has no nutes. Bottled water is sterile except for trace minerals in some. And tap water is usually chlorinated. So if anything your "flushing" the nutrients out of the dirt. Not putting new nutes in.
And I've never heard of any 72hr darkness methods either. But again, outdoor and no flushing.