Okay, working at a karate school i get huge rants from my boss (and teacher) about how to explain to the kids why we set our hands after we block. From a "tell a kid so it makes enough sense to do it" standpoint we say "because the block is finished and the person can't hit you anymore" from a physics standpoint (and how i taught the teen adult class). The blocking motion in front of your body pivots your pelvis and upper body alone your spine. This is just like a sideways teeter-totter from preschool. now when we set our hands, the hips and shoulders pivot back. so when we bring the hands back, AND punch right afterwards, the force behind the punch isn't just that of the punch, its also the force of the block being set and blocking at the same time. Thus giving more force behind the punch AND more speed to set the hand and get it out of the way before the opponent can grab it and go from distanced hand to hand to grappling.