you don't want to trim during flowering. Thats all. The plant has to recover after you clip shit. Which means while it's not recovering, it's not growing buds. Thats the big deal: not growing buds. You veg, shape your plant, then put full forces into buds. I trimed the shit dick out of my current clone. Of course, my build for this is a lollipop, so i'm supposed to trim the bottom leaves anyways and encourage massive undergrowth, which, so far, i love what it is doing. I just hope it still yields...
But still. You just don't want to trim off things the plant is actively using. Dead leaves, trim those suckers. It's recommended. The plant knows how to grow. When leaves stop being used, it kills them. When it send toxins to leaves, those areas of the leaf arn't being used anymore, and if enough damamge is there, might as well clip the whole leaf. It wastes energy into something that's not producing it's fair share.
Don't trim during flowering is the biggest thing to not do...and don't be freakin stupid. Always avoid being stupid.