dude, you do it in veg from early on then stop a week before flower. the purpose is to disrupt top shoot growth so the plants redirects growth to the lower shoots and bud sights that usually don't get light in flowering. your plants should look like a Jewish menorah instead of a Christmas tree....Hmm, i just realized the humorous contrast in my previous sentence

you need to look at your plant in veg and visualize every bud sight being a huge frosty bud in 8-10 flowering weeks, and don't fuck with them during flower cause you will shock them and stunt growth!