Flushing is mostly needed for those that have been pushing their plant to its max with nutrients. Some methods do not necessarily require nor recommend a flush, Lucas of the “Lucas Formula” does not necessarily believe in flushing or has said something similar to that effect. However, the Lucas Formula is a very low dose nutrient formula and the idea behind flushing is not about flushing nutrients out of the plant and it is kind of deceptive or confusing to some. You can only flush nutrients out of the soil or the ‘growing medium’, once in the plant it can’t come backwards out of the plant out of the roots. Plain water can be used for soil flushing in most instances, however they do make additives that can supposedly help dissolve built up nutrient salts, some people swear by these and some say they do absolutely nothing for them.
The real idea behind flushing is kind of starving the plant the last couple of weeks or few days it has during the flowering period by only giving it nutrient free pH balanced water. This is what causes the large fan leaves to yellow as the plant then starts using nutrients stored in these leaves as there is nothing new coming up from the root system. This process potentially can make the end product more refined, maybe better flavor and smoother to smoke and it might make the curing period shorter or easier with a overall better result, by having the plant use up other stores of sugars and nutrients stored in the rest of the plant’s tissues. So the length might need to be longer with bigger plants as they will have more and larger leaves and much more tissues that may be storing these nutrients and sugars.