from what i know it also helps the plant reach the end of its life, watering with plain water she'll just be drinking nothing but it and she will use up all the nutes left over in her system,
Trust me if you haven't smoked bud that wasn't flushed/ or flushed long enough it tastes horrible its very harsh and will give you a head ache from the nutes left over in it, really ruins the smoke,
im only a newbie but i'd flush that for 1-2 weeks then harvest, it should be prime. but then again i could be totally wrong so wait for one of the pros to come along and give input