Contrary to popular belief, budz420's descrition is entirely correct, but flushsing your plants may not be the best thing for them. Starving plants of nutrients during the last two weeks of flowering can hurt bud production that is supposed to grow the most in the last two weeks.
When I have had plants that were really dark green toward the last few weeks of flowering, using tons of water and no nutrients did not do much for the dark green-ness of the plant. I did notice signs of nutrient deficiencies however, leading me to this conclusion.
A better plan may be to map out nutrient usage and make sure there are less excess nutrients in the soil toward the end of flowering. This way, the plant does not suffer in the final weeks of flower and flavor is not compromised with harsh chemicals.
I hope my babble is coherent.
-greatherb