Defoliation is a method to allow light in the canopy in order to let the smaller buds get light to grow, day 21 of flower is usually the last time you want to defoliate last week of veg and then day 21 is what I did and I pick off here and there the ones that cause a lot of shade to the lower branches...I clear off anything lower the the 4th node down the stock to let the tops really get all the major growth and not let the little branches leach off the plant...