This is from a cat that went by the handle nmeeks. Thought he did a really good job at explaining this.
Sources vs. Sinks
First of all, you will need to understand the concept of Source vs. Sink within a plant. Pretty self explanatory, a source is any part of the plant that generates more photosynthate (sugar) than it requires for growth, and a sink is any part of the plant that requires more photosynthate than it can produce (or is producing). What is key to understand with this is that a plant tissue can change from sink, to source, and back to sink all as part of the plants natural process. For example, a newly forming leaf (fan leaf or bud leaf) is always going to be a sink, requiring more sugar to grow than it is currently producing from photosynthesis. . . but once that leaf has reached close to its mature size, it is producing much more sugar than it requires because it is hardly growing at that point. . . .and then again as that leaf becomes older and cells start to get older, the chlorophyll will actually lose effeciency and although the leaf may still look green and healthy, it is no longer generating more photosynthate than it requires to stay alive, and therefore older leaves become sinks again!
There are also parts of the plant which will almost always be sinks. Those are things like the roots (obviously no way of producing photosynthate, but still require it), the flowers on most plants because most flowers contain very little to no chlorophyll compared to a leaf, and seeds are always going to be sinks (the strongest sink).
The important reason I am explaining the Source vs. Sink relationship is because when you have too many sinks and not enough sources, your yield goes down and the overall vigor of your plant is reduced. So to maximize yield and sugar content in the final product (the buds), growers should try to eliminate other sinks as much as possible, while maintaining as many strong sources as they can without risking poor airflow or reduced efficiency with too much shading.
Keeping all this in mind, you can guess that a leaf which is receiving less light (heavily shaded) is most likely going to be functioning as a sink in the plant, drawing sugar away from other sinks that you may be more concerned about such as the buds. Removing leaves above the shaded leaf may give it more light, and maybe enough to start generating more sugar than it needs, but the lower down the leaf, the sooner it will be past its highest efficiency and the closer it is to becoming a sink permanently because of photosynthetic efficiency loss. Therefore, I tend to remove older fan leaves (less efficient) first before I go removing mature newer leaves higher up on the stems because they are more efficient.
Now, once you have removed any leaves that are past their prime, you may still have some fan leaves shading other bud sites, which in cannabis do produce their own smaller bud leaves. The larger fan leaves are MUCH more efficient at producing photosynthate than the smaller bud leaves, and because they tend to be more exposed to air flow they will also transpire more, meaning they help more water and nutrients move through the plant compared to bud leaves which have much less surface area and transpire/photosynthesize much less. So, that means it is better to keep large fan leaves, even if they are shading a bud site or two, because they will function as a source for the bud site (sink) and send its excess sugar to the bud. If you remove the fan leaf, the bud is already functioning as a sink, and so will have to get the rest of the sugar it needs from a different fan leaf on the plant (different source required), meaning a different sink is now getting less than it was because it is sharing its sugar supply from its own fan leaf. . . . this kind of sharing and relocating of sugar pathways takes extra energy in itself, and is not beneficial even if the bud site is no longer being shaded and can produce slightly more sugar on its own from the small bud leaves.
In conclusion, remove old fan leaves especially if they are no longer receiving any direct light. Try to maintain as many sources on the plant as possible by getting as much light and air flow to the newer mature fan leaves as possible. Remove any very low sinks that are far away from any strong sources because they will generate almost no photosynthate on their own, along with drawing photosynthate away from other sinks that are more local to the source leaves higher on the plant. And don’t worry if your bud leaves are shaded, because they are sinks anyways, so the plant is pumping all the excess sugar from local fan leaves to the nearest/strongest sinks it can find (your buds)!!
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. Tomorrow, if my back will allow, this bad boy will be going up 
