It hurts them, as they are the solar panels of the plant and help the plant use the light energy to grow. Dont trim them unless absolutely needed. Show up some pics of what you are trying to trim.
i tried a trimming technique my friend told me about, he says trimming a certain way will help the plant to become more bushy rather than tall. Well i tried that and know my plant is basically a fucking lowryder. It has grown less than an inch in a month