I think these posts touch on great ideas for understanding the feel of the form better. Below is my "take" on them.
I think that the traditional classification that most closely corresponds to this discussion is "opening" and "closing," which are criteria I find very helpful in many postures, especially in trying to focus on "storing and releasing" energy and in keeping the energy flow continuous. In some other postures, however, I do not find this opposition particularly helpful, at least on a macro level.
For example, in Brush Knee and Twist Step, it is not obvious to me which stages of the movement are "open" and which are "closed," even though the status of a particular arm or leg may be clear. In analyzing and organizing the feel of the form, I rely on other criteria.
<font face="Verdana, Arial" size="2">I group them into forms where body parts extend in opposite directions, move in the same direction, and unclear directions.</font>
I would suggest modifying this three-way classification into a slightly different system. What I do is try to use opposition to create harmony, while occasionally supplementing by direct external harmonies.
In terms of opposition, I feel in the form for vertical opposition, front-to-back opposition, side-to-side opposition, diagonal opposition, spiral opposition, end-to-end opposition, inner-to-outer opposition, and complex opposition.
Before I explain my terms, let me address why I find the idea of opposition helpful. As I understand it, Yang Style Taijiquan (and other styles as well) generally takes a natural approach to things. In nature, cause and effect is often quite subtle and indirect. Often, nature works through a dynamic equilibrium, where opposite tendencies combine to produce what appears to be a third thing.
Consider drawing a circle with a string and a pencil. You put the finger of one hand on one end of the string, and you tie the other end of the string near the tip of the pencil. You then grab the pencil with your free hand, use the pencil to pull the string taught, and then draw the circle. As you draw the circle, you must concentrate on two things: holding the string taught and drawing in a straight line (i.e., a tangent), not in a curve. If you try to draw in a curve, rather than in a straight line, you will lose the tension in the string. A loose string will make the radius of your circle vary and result in a wiggly line and a wobbly circle. The seeming paradox is that you must try to draw a straight line in order to end up drawing in a perfect curve.
I find the concept of opposition important from another perspective as well, which is the way it helps "relaxation." According to my understanding, the term "relaxation" covers various aspects that have different prominence in various parts of Tai Chi. In the Association's form, I would say that loosening, extending, and opening the joints is the most important aspect. If you feel the physical oppositions in the form, this feeling can greatly aid in learning to loosen and extend. It feels as if the movements themselves help you to relax and unify the body.
If I return to my classifications of opposition, I would say that different postures make different oppositions prominent; however, in reality, I think all of them are present in every posture and in every transition.
I focus on this during the Opening and Closing Postures, White Crane, Golden Rooster, and during the kicks. In the Opening and Closing Postures, the opposition is mostly along the spine as I focus on pushing up the crown of the head and pulling open the lumbar area (i.e., relax the waist (song yao)). There is also vertical opposition in the movement of the arms. In White Crane, we open up and down, rather than side to side, and so the vertical opposition can include the feeling of pushing up the palm heel and edge of the top hand in opposition to sinking the shoulder, torso, and pelvis.
Here I might include High Pat on Horse, Thrusting Palm, Push, Fan Through the Back, Needle at Sea Bottom, and Chop with Fist.
I feel for this constantly in order to "sink the shoulders" and "round the back" in all postures, especially when the torso is square and the arms are uneven (e.g., during punches), but I also feel this in Cloud Hands. In Cloud Hands I feel that in the top arm, the back, shoulder, and elbow should pull the forearm and hand along. I feel as if I am trying to leave my hand behind as I "pull" from side-to-side with my waist. I also feel this opposition in the transition into Cross Hands.
Here I might include Ward Off Left, Parting Wild Horses Mane, and Diagonal Flying. There is also frequently an opposition between the arms and the stepping foot as it touches the ground. Examples include closing the arms to the left as we step right into Diagonal Flying, reaching to the right and stepping left in Strike Tiger Left, and circling right and stepping left in Brush Knee and Twist Right.
Here I mean where the arms feel that they are chasing each other in spirals on opposite sides of a wheel or grindstone. Examples could be Play the Pipa, Cloud Hands, Step Back to Ride the Tiger, or the transitions into Ward Off Left and Right, Separate Foot, and Lotus Kick.
Sometimes, the body and the application do not allow for an opposition that is geometrically straight; nevertheless, the opposition can clearly be felt. Here I would include Single Whip, the arms during the kicks, palm heal to palm heel in Push (along the back), and palm heel to shoulder in the right arm of Roll Back.
Here I mean expanding the body into a large circle or sphere. Examples might include the arms in Cross Hands, the top arm in White Crane, Press (with the left elbow a little lower than the right), and any of the Ward Off arm shapes in Roll Back, the kick transitions, etc.
I would use something like this term to cover those situations where the oppositions are combined in such a way that it is hard to separate them out simply. Lifting Hands is an example where I feel oppositions through my back and long curves in my arms that are not easy to describe.
By feeling for all these oppositions, I feel that I can loosen the limbs and unify the body more thoroughly. It also helps in defining how movements should be coordinated. For example, if I concentrate on the opposition between the right hand and the left foot in Brush Knee Right, this defines exactly when I should step. Yang Jun talks frequently about coordinating the final closing of the arms with the foot touching the ground at the end of a step.
The only times I explicitly concentrate on "same direction" movement is when I feel lifting the foot needs to be coordinated with the movement of the arms. Examples might include lifting the right foot and moving the right arm in Lifting Hands, moving the right foot back and beginning to move the arms back on the left after Chop with Fist. I find this last transition particular interesting and difficult because although I think you draw the right foot and the arms back together, you still have to reach a position where the arms are at their maximum to the left and back as the right heel extends and retouches the ground on the right.
<font face="Verdana, Arial" size="2">From wuji to qishi, on raising, most just raise the hands without bending the knees! I was taught to just raise hands also but exposure to other than my main teachers, I see value (i.e. function) in bending knees to go 'under an attack' so I incorporated the bending of knees. </font>
I was originally taught to bend the knees, but now no longer do so. I think which is best depends on what you want to train and what your "intention" is for the posture.
I think of the the Opening Posture ( Qi Shi) as training us to do four things: (1) to open the body and rouse the Qi as we stretch all the limbs, (2) to move the Jin upward, (3) to execute a particular application without preparing anything independently of the opponent, and (4) to respond to aggression in appropriate measure. For these purposes, I would not want to bend my knees.
If I want to stretch all my limbs, I want to stretch my knees as well, and cannot bend them. If I want to move Jin upward as I do in the kicks and Golden Rooster, then I must have them same straight leg as I have in those postures. If I immediately want to change the energy in the opponent through my wrists, then my first motion must be in my wrist and the rest of my body must support this motion immediately. Lastly, if the opponent has shown only minimum aggression toward me (e.g., he grabs my wrist with the intent of holding me in place), I should show only an equivalent response (e.g., freeing my wrist and pushing the opponent away). Merely bending both knees and sinking can signal a very aggressive intent and trigger a confrontation. Standing loosely on the balls of your feet is more ambiguous.