Flushing is a weird term, a few years back it referred to what you did when you had a chemical salt build up mainly in rockwool slabs but also when over using chemical ferts in compost. ie running lots of water through the medium until all the built up fertiliser and crystallised salts were washed away.

Now it seems it also refers to starving plants during the last week or two of flowering. This of course gives an inferior product compared to what each plant is capable of producing within its genetic potential, ie: less psycoactivity and potency, much more CBD and CBN, less THC, less turpenes and finally lower final crop weight.

Some people in the hydro world now think that by running on just water that they can flush out excess chemicals from the plant by some form of reverse osmosis through the roots into the flushing water. Its amazing the myths that grow in the cannabis community.

Outdoors when growing in soil, where cultivated organically it is nearly impossible to remove any nutrients by rain or water running through the soil. It is only with modern farming practices, that huge amounts of chemical salts get flushed/washed from the soil into our water courses/rivers every time it rains. This happens with both chemicals and through the bad practice of slurry spreading.

The major expansion of bud growth happens at least a couple of weeks before they are ready to harvest, you can see when this has happened, after that it is better to have a ratio with lower levels of N and P but higher levels of K and of course lower levels of feed would be needed as the plants metabolism is winding down. Excess levels of residual P make the bud harder to burn and give a nasty taste in the mouth. Also it make the smoke many times more carcinogenic. Bud with low residual P burns to a white ash especially if it has a reasonable level of K, the more P the greyer the ash and the harsher the taste when burnt.