Lemonboy has it right EDIT: and J Dog... its right when you cut the lower branches off... there a a couple of reasons of doing this...

- instead of the plant working on the lower bud production on parts (bottom)the light barley reaches anyways, the plant can focus on the main and central bud formation and packing some heat in some instances

#2 - Healthier envirement to prevent mold/fungus/and spider mites... put a fan under the "open" area now, and will drastically help prevent mold...

#3 -- dont the plant branching out as much

However some stranis take this harsh and actually reduces the amount of grams you can achieve