defoliation during veg is a definate no no, but i have seen some evidence that removal of old fan leaves half way through the flowering cycle can increase yield, however i cant say from experience if it actually works. the theory behind it is that the old veg leaves are no longer feeding bud growth as that job is being fulfilled by the new sugar leaves that grow as the bud develops and that the large fan leaves end up using more energy to maintain themselves then they produce. I have been reluctant to try it though as i only do small 4-5 plant grows usually from seed so any difference in yield could be because of different phenotypes. but i cant deny the evidence ive seen has made me want to give it a try and ill likely give i a shot on my next clone only grow, but that is still several months away as im currently auditioing for a new mother plant
Owning my noob-ness I have all of 3/4 of one grow to build credibility. For this first grow, I decided that I wasn't going remove leaves. Then during about 4 weeks into flowering, I noticed that some of the older fan leaves were yellowing and losing their turgidity. Growing a SCROG in a greenhouse at 45 degree angle (toward the sun / windows). Well a succession of sunny days can get the temps up and even though I have a fan pointed at the back of the angled SCROG, the scrog is pretty thick and reducing air flow. So these limp, faded, pale, yellowish old fan leaves block the air. So I changed my mind and have been pulling and clipping these leaves off.
My deduction was that these leaves weren't contributing much to the immediate energy production or needs of the plant.
Now I think mother nature knows a bit more about how to grow than I, so if these yellowing leaves really were useless, wouldn't they fall off? In fact some of the leaves did come off really easily with a gentle tug. But maybe these old yellow leaves were designed my mother nature to hang around as an emergency water/nutrient source for the plant when it meets severe drought at the end of it's seed cycle. So since I can ensure these plants don't get hit with a drought, then maybe there is nothing to lose from trimming these leaves.