A tip from My time in China
During a brief practice before breakfast, Shifu commented on my six sealing four closing. He said in this move the front arm is creating space as we open the kua. When we create space, we must take that space over, filling it in. This is our advantage over our opponent. If the opponent fills the space in before us, it becomes his advantage. So, there is a subtle move of the torso on that side to fill in that space. If we move too much, we are moving the center. If we move too little, we leave an opening for our opponent. For many of us, including me, we often tend to move slightly back when we open the front kua, back loading. This gives even more space for our opponent to fill in. So, it will take some hours of practice to make the proper move automatic.