Kua (kuà - 跨) is a term that is usually translated as hip. In my practice I usually refer to the kuas as the area where the upper leg attaches to the abdomen (inguinal fold). A tai chi teacher will keep reminding the students to relax the kuas. It means to loosen the hips to allow the kuas to open and close as we move from side to side as the spiral force moves us.
Why should we care? It's all about the shoulders (among other things). Beginners use the shoulder muscles to move the arms. Think about something like Cloud Hands. It doesn't matter what style. Step out to the side, shift the weight, move the arms. All separate actions.
With more practice and instruction, the student learns to use spiral force to rotate the dan tien and open and close the kuas. Read more about spiral force here. The kuas are the linkage that transfers power from the ground to the upper part of the body.
Now the motion of Cloud Hands is different. Step out, spiral the force from the ground to rotate the hips, use that force to rotate the dan tien and the rest of the abdomen, and use that motion to pull the arms along with the weight shift.
Cloud hands becomes a whole body motion, with relaxed kuas and shoulders. It's a very different kind of motion. And it started because of relaxed kuas and force spiraling out of the ground.
© 2013 Eric Borreson