Well, the way it was explained to me, (and this guy's garden (the one who told me) is amazing - to put it lightly) was that: At The point where the stems turn purple, they are actually not benefitting the plant anymore (actually sucking more light and nutes away from buds) and wasting it on the leaves. some might disagree, i had always thought that the fan leaves "fed/supplied" the plant, but he told me that they take away from the plant _or what you want it to be doing for you, once the stems turn purple, they pick off easier, maybe thats why??? All i know is if someone is successful, never do what they say, rather do what they Do.