Who says? I top and do a heavy defol if necessary. My current DWC grow (Blackstrap BE) I have been taking 5-10 fans a day during late stretch. My previous grow (AvT) same thing - ended up with a 1 lb. plant. If topping (<day 21) and defoliation (spread out as necessary) are done correctly it will increase, not decrease, yield (not to mention its beneficial effect on preventing bud rot, etc.). IMHO, YMMV, etc.
I try to cut fan leaves that make their way to the top from lower on the stem and leave the fan leaves around the edges like a skirt. My test is - if after defoliation you can look down from the top and see green (no gaps) it's safe to defoliate upper fans - idea here is that a photon will have to hit
some leaf on its trajectory downward. Here's the Blackstrap after yesterday's defoliation:
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