I'll try to video something with a student to illustrate my point. If my hand has come forward into the shape you describe if it makes no contact with anything it is going straight into the target with no hesitation, this in itself will diminish the power in the incoming strike. Secondly my wu sau is already forward, maybe you have a different concept of wu sau structure, if you train to hold the wu sau very far back and not forward of the elbow then i agree it may collapse, we don't train that way so turning the Wu into another shape is instant and happens concurrently with the forward thrust of the other hand. Finally I assume that you have turned / angled with the intial response? I understand your claim that stepping may be out of the question given the speed of action but a simple change of angle would always be part of my inital movement which moves my head away from the intended target area thus making the initial strike less likely to land and also providing more space for the wu to transition. With regard to compromising the centreline every time you do pak sau, I can only say that you must do pak sau very differently to me, the only time pak sau compromises the centre in the way I do it is if I apply it to the inside of the arm which I only do if either forced into a last ditch recovery (ie my other arm has been taken out of the equation by a grab or similar) or if the opponents other arm has already been taken out of the equation. I'd equally question your logic regarding the front arm being close to the punch whilst saying the wu sao won't have time to change shape. If the punch has set off and you've misjudged it then if its got anything like reasonable speed and power it is almost certainly already past your lead hand.
Only a different opinion of course, as I said i'll try to video something as we may be talking at crossed purposes.
I don't think you will be able to pivot either , if I'm right in front of you with both my hands an even distance from your head , you will only have a nanosecond for your brain to process the information and decide which way to pivot.
He only has to move one limb you have to pivot your whole body one way or the other.
In stepping forward you only have one decision to make , that is move forward as soon as you see movement.
The point of contact for your Tan Sau will be somewhere on his upper forearm , because the horse has already bolted past the point for wrist contact.
You say that your front hand will just continue in and strike the opponent , thats fine and dandy if the opponent has roughly the same reach as you.
But if it's a tall dude with long arms , his strike will probably get to you first , which brings us back to this little problem of his fist about to smash into your cheek bone.
The fastest movement you can do with the back hand one that is more in line with a natural flinch response and one that is more likely to happen under pressure is for you to simply raise your Wu Sau a few inches to provide a barrier to your face.
In regards to Pak Sau I normally parry the outside of the arm as well , but to recover from an error I will parry the inside of the arm.
If you say you have your Wu Sau forward then that means you will have both hands forward against straight punches , which leaves a great big gaping hole for any uppercuts he might decide to do or a throat grab.
You are correct in that the mistaken counter is two movements , but they are two very fast minimal movements that allow you to get at least one of his arms under control and trapped very quickly as you strike through.
