from - http://www.growery.org/2947/Sexing-your-plantsIt is possible to tell the sex of a plant early, and thus move male plants out of the main growing area sooner by covering a plant is lower branch for 12 hours a day while it is in a constant light vegetative state. Use a black paper bag or equivalent to allow for air flow while keeping out light. Be sure to set up a regular cycle for these covered branches. If light is allowed to reach them during the dark period, they may not indicate early at all.
ok thanks for the infoIt will only be the branch that you cover that will flower. The rest of the plant will continue to veg. (at least that's how I understand it)
It's a way of sexing a plant without flowering it.
from - http://www.growery.org/2947/Sexing-your-plants
Just had a little look around and that is the theory for sure. It should only flower the covered branch.