I have read here and elsewhere that open and close are present at every turn in Sun taiji. I am sure this is true, but I feel that this concept and these actions are much more important than this statement expresses. I hope to get some comments especially from Tim who is the only person I know of with direct contact with Sun Jian Yun's family taiji and also highly regarded by Doug Lee a teacher I respect very much.
My feeling is that the reason that Open and Close get top billing in Master Sun's own title for the form (Kai He Huo Bu taiji) is because opening and closing is essential to every move, just as the 4 word formula is essential to wu/hao style.

I base this solely on my own study of the form, which is learned through videos of:
Jiang Jianle
Liang Shou Yu
2 sets of Sun Jian Yun videos
various chinese vcds of competition sets

I'm sorry to say I've never met someone who I truly trusted to teach me the form, because it was always some extra dish in their wu shu buffet. I was often told I had to learn the chen or the yang style first as if learning one system would teach me a second. I'm sorry to say this aspect of the modern taiji business disgusts me. So, a lot of my theory comes from my Yi-Quan teacher, because like most of my teachers, he proved that he could use the techniques he taught and because he believed 100% in Yi-Quan

However, back to my point. Sun has been the main form I practice for several years and I've come to the conclusion that the title of the style describes what takes place in every move It is in itself Sun's 4 word formula, because the active stepping is also a form of opening and closing that involves a constant expansion and contraction of the body. I believe that this method of power delivery makes it also the soundest style of taiji because its essence is mobility. Ihere is nothing to touch but the rebound is with total body weight

If I sound like a sun chauvinist, sorry all you yang and chen and wu guys I know your styles have much to offer, but to me the footwork of sun style puts it ahead of the pack. Sun Jian Yun's pace in her seventies (the first set of videos I possess is fast yet always relaxed, composed and full of intention) it is a combat pace though light (I wonder if the Sun Shu Rong branch of the family is so light in execution) Her pace in bagua was a lot faster at a later age.
The forms of the sun set lend themselves easily to a fast pace and fajin is easy and natural to add in. this is not so true of the other styles slow sets though they practice at speed in push hands and 2 man sets. However so many people do not even learn these aspects of the style and most who know push hands in America seem to know only stationary push hands.
The best way to learn something is to practice it and combat requires movement. The classics also say look to the footwork.

Well I guess I've already shot my mouth off more than I should, but I look forward to your comments . I have certainly enjoyed a lot of what I've read on this board. (except for the thread destroying attempts at humor.)
Cheers all

The two opposites are essential to everything. You can't have one without the other.

From my limited understanding of the Sun-style (like Mr. Crilly, I was self-taught but have had the pleasure of being corrected by Tim in recent months), Open & Close are means of centering one's balance periodically throughout the form; as well as important aspects of a variety of martial applications; as well as what Meynard so aptly pointed out (and in fewer words) a key aspect of any movement in the Sun form/applications.

Good luck finding a real teacher of this system, John, I know that it was eye-opening (and back to the drawing board) when I experienced Tim demonstrating the principles of movement that are inherent in Sun style. You can't beat personal instruction.

As I understand it the open and close movements are for gathering chi when a directional change happens in the form.

When you are in a combat situation, gathering your balance must include gathering in your opponent or you will not be in position to control the situation.
I believe that is what is meant by connect in Wu/Hao taiji. For those who do not know it the 4 word formula of the style that Sun Lu Tang adapted is: Begin .. Connect .. Open .. Close I wish I knew more than that
It doesn't seem like Sun retained the same strategies, however. and here I find I'm confused. It would be nice to know a practitioner of that style in B.C. anybody know someone?