You don't intercept the jab specifically. Wing chun's response to any attack is simply throwing two simultaneous punches and a kick from the center, then stepping forward. Any direct attack, including jabs, will probably make contact with your hands before your head. Then you have to feel the vector of the attack and move your arm accordingly, doing tan sao, bong sao, whatever is necessary. It sounds far-fetched, but it gets automaticized after a lot of practice, and there is the kick to keep the opponent at a distance in case the attack comes from an angle, so you're relatively safe.
The wing tzun straight punch will wedge a straight punch and deflect it. The jab is thrown to full extension and that’s why it’s a straight punch. For the sake of demonstrating the tan sao, the partner fires a bent arm punch. The bent arm has more leverage than a straight arm (simple physics you can test. Hold your arm out straight and have some one push it up or down. You’ll see it’s difficult if not impossible to keep your arm from moving. Then, bend your elbow and see if the partner can lift your arm up or down. You’ll see it becomes drastically more difficult to move your arm👍) if the partner fires a jab or wing tzun straight punch, he would just punch simultaneously while moving in with forward pressure and the jab or straight will be deflected along the shoulder line. It’s like hitting a door stop under a door. You need a wedge (the arm shape of the straight punch) and a pressure (forward motion) to stop the door. If the elbow is down it has more leverage than the wedge punch. So, the energy is suppressed and instead of trying to fight your partners leverage and strength, you allow the arm to deform into the tan sao shape, allied with angular foot work. Touch is faster than eyesight and is a more primitive sense and part of the reptilian brain stem. Forming the tan sao allows the opponent to still feel like he is touching something and is safe without feeling the line of attack has been changed (you have to experience this directly from a good student or a teacher as it doesn’t carry over very well visually on film but if it’s done properly, you feel as if the opponent is still directly in front of you when in fact he is in a different angular position and able to attack with two arms to your one) in short, I’ve boxed for years in amateur and I wouldn’t think of trying to jab my instructor, he would blow my head off. Even if you retract quickly, the wing tzun man will still wedge and feel the arm retract (we have drills for this too). Interception is a different pocket of timing to say a parry. When the arm with draws, the wing tzun man will stick (this is one of the first questions I asked my instructor and he demonstrated with no uncertainty at all that he could demolish me even if I retracted my arm very fast. This is what actually made me sign up! I thought ‘if he can beat my jab even if I flick it out and and fake him with it and still put me in a bad spot, I’m sold’ he did and I have been hooked ever since! Don’t take my word for it, go and check out an EBMAS wing tzun school and see for yourself. For me it was a humbling and enlightening experience) Any ways all the best champ!
