I don't claim any skill at Chi Sao. In fact, the only thing I claim to be an expert in is being me. That said, I'll throw my two cents worth in on Chi Sao here. I was introduced to it under a guy named Kimo Wong who taught it in Chinatown in downtown Honolulu, in some run-down building near King Street back in the mid-70s. Yeah, I think Jack Lord was still filming Hawaii Five-O back then--it was a while back. I think Kimo was one of James DeMile's students, and DeMile was one of Lee's original Jun Fan people. Kimo was a little guy, but he hit really hard. I watched the Chi Sao stuff being taught to more advanced students, though I was never really formally trained in it being the new slug in the class. In those days, being a new student at the kwoon, I was usually already exhausted by the three hours of station-training "conditioning" that Kimo put us through. I was far too tired after that to even move, much less learn advanced stuff. So, what I say here is mostly my "impression" of Chi Sao and Wing Chun in general.
I am sure it has an application somewhere. Kimo was very fast and could tie your arms up in a second when trapping. Chi Sao seemed more esoteric, like Push Hands in Tai Chi--neat idea, but how do you use it? But Wing Chun seemed to depend a great deal on straight-line blast attacks, which, considering it came from the boat docks of Canton, is not all that surprising. It works fine if you can get in close and tie up the guys arms and punch him backwards so that he cannot recover. It works against people not trained to deal with it. Keep in mind, a master can do a lot more than the average joe, but even they get defeated at times (back in Texas we'd say "there ain't no horse that can't be rode, and there ain't no man who can't be throwed.")
It seems to me that once a trained opponent got outside of the straigtht blast angle and hit it from the flank, the Wing Chun practicioner had to reorient hsi centerline quickly or be at a serious disadvantage. Hitting them 90 degrees to the blast really knocks them off balance, at least the lesser skilled ones.
In terms of martial "art," where this is a replicated traditional art of Hong Kong, I can buy it...sort of like how I can buy traditional Thai Dancing or Noh theater. It is an art. it isn't living, breathing spontaniety. In terms of a combat science, it has its good points, but it doesn't cover the whole ball of wax by a long shot. It is designed primarily for street fights using fists, more approrpaite for a duel than combat. It is not meant to be an armed combat method.
Even so, I have serious reservations about some of what I was taught. I remember being taught the Chung Choy punch, the one where you punch straight ahead with the bottom knuckles of the fist. It works if you hit a soft enough target and use a whip-like snap rather than body weighted force. But one time while experimenting with using that style of punching with a karate-style hip-twist, I easily broke my hand, even though it was encased in a padded glove. The structure of striking with the bottom knuckles is just too weak for any real force to be used, and particularly dangerous if striking a hard target like the head (read-read my dislike for punching in general in other threads. I have learned how to hurt myself pretty good with a closed fist). Structurally, the karate-style punch is much stronger, but I still have reservations about striking my knuckles against hard surfaces a lot (as a guitarist, I really need to protect those hands, particularly the ability to articulate my finger joints. Having broken metacarpals on each hand once when punching, I have decided to move away from that method of striking if possible).
I think in terms of "art," the circular style of Pa Kua is probably far more sophisticated than Wing Chun. The footwork is certainly more impressive, and not as subject to being knocked off balance. And I think because of its circular movement, it woudl give a Wing Chun man a tough opponent if two equal practicioners were matched. At least as far as "arts" go.
But look at these words: "matched." That denotes a duel or sporting type of event. This is where all of the arts get into trouble with combat. Real combat is no sport. You cheat as often and as hard as you can. He takes a stance, you grab a bat. He pulls a knife, you pull a gun. Real criminals kills people every day like that with very little training, and sometimes they kill even highly trained sport fighters. I think this is where we often get into confusion. If we're talking art let's talk art; if we're talking combat, then it's a whole different game. The wonderul thing about this forum is the attempt to bridge this gap. But it remains very much an entrenched way of thinking, to talk about one while using the conditions of the other.
I liked Wing Chun. It was fun. But there is a reason it is not taught to SpecWar operators. It takes too long to learn (there's a lesson there), it is too restrictive in its angles, and it doesn't work well with modern weapons. It is about a "style," an "art," not victory at all costs. Parts of it can be useful in less-than-lethal confrontations, but only if you have drilled in it a lot. Necessary if you plan to teach the art; not as much for combat where such hours of drill on one thing is seldom needed.
Even Bruce Lee decided to get away from Wing Chun. If I recall correctly, he was in a challenge fight (duel) with Wong Jack Man in his kwoon--the famous challenge where the sifus of San Francisco sent their top fighter to stop Bruce from teaching gung fu to non-Chinese (a fight the movie totally screwed up, by the way). In the real fight, according to Lee's wife Linda, Wong Jack Man quickly began running away after having taken a few of Bruce's straight fist shots. Bruce was then forced to chase him around the kwoon, and since he was using the Chung Choy style of punching used in Wing Chun, he ended up punching the back of Man's head as he ran away. Not only was this not very effective, it hurt Bruce's hands as well. I believe that is the moment of epiphany (read disillusionment) when Bruce began to away from Wing Chun as his primary art, even though he incorporated much of it into his "no style" JKD . In that context, Chi Sao is just one page in a chapter called Wing Chun. Neither of which is the whole novel.
All of the comments about Bill Cheung I find rather disturbing. If he is going to teach Wing Chun, teach it as an art and let it be. There's no need to go after the combat crowd. Why do they always have to go for notorioety, the fast buck, and the latest fad? Whatever happend to humility and manners in martial art masters?
I guess it's easier to have a philosophy than to live by it.