Generally you "flush" your plants if say you accidently drop a whole bunch of fertilizer in your pot but in this case you just want to remove any chemical residue from being in the plant when it's time to harvest. wait about a week to two weeks to flush your plants. just water them with some good water. You'll want to test it to make sure the ph is correct or you can go to Wal-Mart and pick up some spring water or something for like 50 cents a gallon (you might need 2) and just maintain your normal watering cycle. Something I've also heard of is adding some extract, ie: vanilla extract, strawberry etc. a few hours before harvesting and your plant will suck up the extract to give it a little flavor kick. Haven't tried it myself but I am interested in it.
For a good flush. You need to run 2-3 gallons of water through it for every gallon of pot size. you want clear water drianing out of the bottom when this process is done. It will take a while and be very messy. I recommend doing this a couple of weeks before harvest even if there are no problems with the plant. No nutes or salt build-up in the soil(salt is the by-product(****) of the plant) when its harvest time. This will make the bud less harsh to smoke. IMOThe soil will be soaked. but if you pot drains good, you won't need to water for a while. You won't get rootrot as that is caused by constant overwatering and the plant lacking oxygen at the roots. This is a one shot deal and will dry out. The plant might droop a little afterwards, but will perk right up in the next day or 2.
Malitia is right about the PH testing.
I never heard of putting extracts in the soil though.