I've talked to experienced growers that flush their mediums before they change nutrients mid grow... It's a technique that's used in both hydroponics and soil...with a fully certified organic nutrients like the Bio-Canna line from Holland you don't have the same necessity for flushing.. as you are using fully organic material. However, It is still an option for the grower.
Flush or don't Flush.. it can or can't make a fairly large difference.. it all depends.

You got negative rep (from me anyway) for this comment

"no need to flush the dirt, we dont smoke the dirt do we now lol ,,and you cant remove wats already in the plant no matter how many ltrs of water you push through the soil."

Not only was the comment a snide one.. but it wasn't entirely correct in being informative.

You personally can not remove anything from the plant.. but you can help the plant to use up the nutrients it has already by flushing out the medium with 4 or 5 times the amount that you usually water.

Wether or not you do this before you harvest has a lot to do with the type of nutrients you've used through-out the grow and how heavy of a feeding regiment you've had it on.

Elstonite, I've grown. I've also compared between a flushed and a non flushed plant. I gotta say, I prefer the taste of a flushed plant.

P.s. I sell hydroponics and HiD lighting for a living, so i'm pretty sure i know what i'm talking about.