Decandling develops ramification, and reduces needle size. The second push of growth has smaller needles, and increasing ramification allows you to spread the strength of the tree over a greater area, so that each future shoot is smaller and doesn't extend as far. It is a stressful operation for the tree, so you have to be careful that the tree is strong and responds well to the process, and it is very important that you use other techniques to balance strength over all foliage areas so that one part of the tree doesn't get too strong, while another part of the tree gets weak and/or dies back.
When you see those beautiful JBP show trees with uniform strength and short needles, they are the result of painstaking maintenance over the course of years. Let the tree grow freely for even two seasons and you could lose a decade's worth of work - the tree will revert to apical dominance, lower branches will weaken and/or die back, the shoots will start to lengthen and the needles get longer... and in two years you have a bush with no interior growth - and no easy way to get that interior growth back.