If the plant doesn't fade it's loaded with nutrients at harvest and you end up with BLACK ash and melted nutrients dripping out of your pipe. The fade removes all traces of fertilizer from the buds leaving you with white ash and crystal clear glass pipes because there are no melted nutrients to coat the glass. In a perfect world all cannabis plants will be completely yellow at harvest with most of the leaves having fallen off.
To achieve a proper fade you need to stop feeding just a few weeks into flower because you don't want anymore nutrients going to the flowers. Let the plant cannibalize itself. Green leaves are bad. Photosynthesis and plant nutrition does not apply to cannabis because cannabis is a super special plant. Cannabis does not need nutrition or photosynthesis to create the best yields. Starving a cannabis plant makes it better.
One should always remember to flush early and often to induce the fade. Without the fade your weed is going to be pure nutrient tasting garbage. Just the other day I was over at an acquaintances residence and they offered me some of their weed to smoke. I took one hit and BAM! I could definitely taste the calcium nitrate. There were hints of monopotassium phosphate but it was subtle. I also got a hint of potassium sulfate. They used a kelp additive so it had the smell of the sea. I closed my eyes and it was like standing on a beach. I could even hear the Seagulls.