I'm a new member to this forum I know this is an old subject. I had a friend ask me the same question. Since I have a lot of experience with plants I will explain why the yellowing. 1) lack of light 2)lack of water 3) heat cold or hot Temps can cause chlorosis. Hence fall is when leaves will drop due to cold Temps and the plant preparing for winters. 4) lack of nutrients. 5) pests. So why do we see lots of yellowing leaves before harvest. Well most people use a flush tech. Which is caused by 1,2,3,4. I have tasted plants that are not flushed and plants that are. IMO plants that are flushed good tend to have a smoother taste. Also the burn is good and ash turns white. If not flushed you can get some snapping and crackling of the smoke. Also it is harsh. Since there is still salts and heavy nutes in the plant. I know this is crazy but a like to add Epsom salts 4 weeks before harvest. Then on the second week do a complete flush. The Epsom salt (magnesium sulfate) helps the plant absorb more nuts out of the ground and helps boost the plant before a true flush. Just some info. Peace