Maybe I can carify a few things regarding what the Wong Shun Leung people do.
Firstly the taan sau is to remain on the centre at all times,even during chi sau.

In the first form, as was so rightly pointed out by Mr Leung there are three sections and three different versions of taan sau.

The first is all to do about Lat Sau Jik Jung. Free hand thrust forward,The whole idea is to train the elbow and not the hands, when you do chi sau you train the forearms and not the hands, you should be generating all your force from the elbow. Try to tighten up your fist and then punch, then relax the hand throw the punch again,and just prior to contact close the fist. Being relaxed in the forearm will give you an advantage in Chi sau as you will not have to consciously think as the arm will be guided to form another structure eg Bong, Taan, jaat sau etc.

When doing the Taan you are training the lat sau jik jung that is as soon as pressure is released from our arm it will spring forward, with out even thinking, if the elbow falls out the hand will follow its same line, and most probably miss the target. So when doing the first section of Siu Nim Tau it is the hand following the elbow and not the elbow following the hand.

Try as a drill to form a taan sau, have a partner press against the taan sau either from on top, either side or from underneath. The person doing the taan sau should have a springy forward pressure. Now let the opponent remove their hand really quickly, your hand should end in his face or chest, not of to the side,top or bottom.

If you are always appliing lat sau jik jung when doing chi sau there should never be a problem, we all attack off the fook sau, and the skilled students deliver a verticle palm straight back into the attackers face, hence the elbow of the verticle palm will over come the fook, which you apply from the first section after the paak sau.

Bong Sau for us is a little different, we maintain that the upper part of the arm lies along the centreline, so when the elbow is truly rotated, then the taan sau elbow will be on the centre again, and not off it. Some people use the hand or fingers on the centre and lift the wrist until about nose level, as they say that the bong can be punched over the top of, this I believe is a dangerous practice as it is quite easy to get underneath and unbalance your opponent, but also make counter punching from the bong very difficult. Using Bong the way we use, makes it easy to come forward and attack with the rear fist and apply a quick trap using the paai jaan principle of Chum Kiu. Some people say that you can only use Bong sau when you pivot, as again you can get punched over the top of, not so, as if this was the case then would it not be dropped out of the Poon sau cycle and some thing else put in, or should it be dropped out of the daan chi sau exercise.
The way WSL Bong Sau is used, is it rotates up and forward, if ever my front hand has been deflected off the centre then I will only use the Bong Sau. Yip Man used to say that Bong Sau was the best techinque not to use. His idea was that the main problem with the Bong Sau is that it does not attack, it only defends, in Wong Sifu`s idea is not to block but to hit the opponent, do not chase or follow then hands but to attack the form.

Everey body talks about the simulataneous blocking and hitting, but many people get caught up in a game of hands, and miss the real picture of belting you adversary.

John

John Smith