Cramping stalks due to getting too tall...

I got a couple plants that just took off over the others. My grow area is 6 ft in height and they've got a foot left to grow before hitting roof top in attic. 400 wt hps light. There is about 6 inches of the tops that has no light shining on them and i don't want to put cfl's on the tops, trying to keep temps down in there.4 weeks into flowering. Someone told me i could cramp the tops of the stalks. How do I do this without snapping the stalk? Do I need to use string to cramp them? Anyone with pictures of this procedure?

You can try to bend the stem down and tie it off to the side of the pot. You really need to consider space while plants are in veg stage. Easier to train when they are younger. Tie a string near top loosely and gently pull the stem down and tie it off.

OK. Yeah. I will top them off next time. Thanks for your info.