from squidbilly can of worms
I found this on another site and this guy does a great job of explaining what leaves actually do.
"My experence Is to top the main cola, wait for that to heal. The plant grows alot of leaves that start in the light soon to be shadowed by higher leaves and at each set of leaves are the nodes that produce tops, that produce the Flowers. I choose to remove with clean sharp scizors all the Leaves on a branch except the top 3 sets. Those sets being one set thats opening still, The set thats still Freshly sprouted and points toward the light still as it hasnt layed all the way flat to the light as the leaves are still sensitive to the light and the angle to the light lowers the intensity on the fresh leaves, and the set below thoses should be fully Developed, and have no sets directly above it. The older a plant gets the more leaves it produces. As these leaves become more and more shadowed they become less and less productive. The plant keeps these shadowed, largely inactive and even dormant leaves for what are known as "Water Leaves". Water Leaves are Leaves that serve as "Water Tanks" for when the plant Experences prolonged dehydration. In such an event the plant pulls the water and nutrients from these large dark dormant leaves. Once the "Water Leaves" are sapped of all there water the leaf simply dies and falls off having served its purpose. These Leaves are in a natural situation are vital to ensure the life of the plant throught the growing season. But In the case of the responsible indoor grower that dosnt forget to keep his or her plants watered they will go completly unused. So the reason that it is good to take these leaves off is the plant will actually expend energy from productive parts of the plant to maintain this "safety net" of Water Leaves. Removal of these will do two things to the plant. the first thing the plant dose is want to replace those "Water Leaves" and its going to do that by devoloping the nodes around ware its "Water Leaves" were near the middle and bottom of the plant. The Second thing that happens by removing these water leaves is light can penitrate the lowest nodes of the plant, which are now being enchouraged by the plant and the presence of direct light contacting the nodes. The third benifit of this strategy is that removal of all the main lower leaves opens up about 80% of the tops on the plant into direct light. The more tops that are exposed to direct light the faster the plant can grow and the more nodes that develope the larger and more abundant the bud production will be come harvest time."
Many fan leaves are competitors for energy, not big 'solar panels' that are the main source of energy for our plants. The plant actually expells energy to keep those leaves alive-not the other way around! So instead of giving your buds energy they actually compete for it. The plant keeps these 'water leaves' alive in case of a possible drought.
We all know what photosynthesis is, well any of us who passed the second grade. Using this term as an example of why you shouldn't remove any leaves demonstrates a common misunderstanding of botany/biology.
Removing leaves selectively throughout a grow, often called 'foraging', is an amazing tool for experienced growers. It boosts your plants immune systems, encourages bigger bushier plants, and redirects energy where YOU the grower wants it.
With a combination of training techniques and leaf removal an indoor grower can maximize his space and yield. There is no way I could LST 9 plants in a 4x4 area without removing leaves almost constantly. There is also no way I could pack my tent with as many colas as I do either without selective removal.
Again, indoors it's all about maximizing your space. I've said it a million times, 20 plants completely defoliated would yeild more then 10 with no leaves removed at all. You might say, "Of course, it's twice as many plants!" Well, that's the point! You can cram way more plants in the same size area. It's not that removing leaves increases yeild per plant, it's that it helps indoor growers maximize their space.
Look at the scrog guys-they remove leaves and lollipop every branch(remove everything under the screen) and they yeild more then anyone else in the same sized area. Both of those techniques get slammed on these boards, but in all honesty great growers have used them for years to increase yield!