Just as one thinks of sticky hands along with Wing Chun, you can also add the center line punch to that as well. Understanding and perfecting the center line punch is a must for those who want to improve in the art. The center line punch is not just a way of hitting an opponent but the mother of all motions in Wing Chun.  It’s key to get this motion correctly or risk damaging your joints if you punch wrong.  Through my experience I'd say 95% or more of the Wing Chun community end up popping there punches.  Therefore, I'll explain the ABC's to doing it correctly, so you can be on your way to mastering it.
When you think of the motion of center line punch, imagine yourself sawing a piece of wood. Thus, its simply a push and pull motion. The key thing to remember is that the elbow is doing all the work, nothing else. Elbow power is from pushing and pulling, elbow leads the motion. Chair kuen means pull and punch.  
In the beginning, we normally start the beginner by placing his fist roughly 6 inches away from the sternum. There are 2 reasons for this. First of all, this gives the beginner a sense of awareness of where his centerline is. (Eventually, this will not be necessary once you know where your center is located). Second, if the fist is to close to the sternum, then the elbow cannot be behind it, in order to push. (Thus, the rough distance of 6 inches from the sternum.)
Prior to the punch, the fist is in a relaxed state, if there’s any tension its only between the index and the thumb pressing upon each other.  And, tension is simply defined as the neutral point between to relaxed or to tight, that’s what it means in the world of Wing Chun.  In addition to that the fist that you are making it should be held at a slight 45 degree angle.  The beginner should not be fixated on having a clenched fist. As Sifu Fong had instructed me, "the more power you do the less power you'll get, you just want the hand naturally in place.   If you force it in the beginning you won't develop that snapping power." In other words, the power is not going to be generated from tension or muscle. It is the push and the follow through that creates that explosive energy.  What happens when you use muscle?  Picture this, if I'm going to run someone over with a car, at the moment I hit him, I don't hit the breaks, instead I keep accelerating through him.  When you use muscle at the end of a punch, its like putting the brakes on before the hit.  Even as I mention this time and time again, many still feel like they need to put that extra umph at the end, which is incorrect.  The way to gauge yourself is the tension you feel with your fist prior to the punch, is the same tension you should carry at the end of the motion.
While the punch originates at the sternum level, it will travel on an incline at the same height of your shoulder. Now this is very important to remember. Failure to do the punch properly can lead to injury to the tendons or joints by the elbow. Remember, when you push with the elbow your simply stretching the joints.  If you punch at the same level, you'll end up popping your elbow. Obviously, that's bad, thus the path is from the sternum and a rise to the shoulder once you fully extend. At the same time, while the fist begins at a 45 degree angle, it ends with a 1/4 turn from the starting point.  The drilling motion is only slight but yet necessary.  Overturn the punch and your initial point of contact will be made with the middle knuckle, which is weak structurally and could break the wrist upon impact.  The rise in the forearm when punching allows for an uprooting motion, when directed towards the opponent.
Now remember that the punch has 2 drilling motions in one. There is the horizontal turn in the punch, as well as the vertical motion of the wrist. When done properly both interconnect with one another. A common mistake by most individuals is to over emphasize the vertical wrist motion. Keep in mind if you do the motion correctly, you don't need to emphasize the drill motion of the wrist.  When you over emphasize the wrist snap, instead of punching through your opponent your energy then sails upward and unfocused.  The end result of anything is a natural motion, all to often if its not attempting to put more power in the punch, it’ll be trying to snap the wrist.  This mentally has to be trained, on not focusing on the power, but focusing on the motion.  Often, I’ve given the example of someone driving a car, if you can maintain the car on a straight path you can increase  the speed and the impact, but if you have no control over that car, you won’t be able to generate the true power.
The final hit may seem as if the bottom three knuckles are making the contact on the target. But in reality, the entire fist hits the opponent, the main force is coming from the 3 bottom knuckles.  To better understand, imagine a nail by itself, if laid onto of a chair, its pretty unstable.  However, surround the nail with cotton and then it becomes stable.  Thus, your fist when it hits is also relaxed, the knuckles represent the nail, and the fist is the cotton surrounding it.  When the fist hits the target, the fingers naturally squeeze in through the force, while the knuckles then drive through to the opponent.  You might be asking why the entire fist? Well, imagine the fist as a triangle, if you were to hit the target with anything but the flat surface of the triangle, the side without the support would collapse. However, if the triangle hits the target flatly, then both sides would be supported.
Here's a list of common mistakes to watch out for:
1) Popping the elbow - This is the number one mistake and serious cause for injury.  If the final motion of the elbow  goes up instead of forward, your popping.
2) Over emphasis in the wrist snap - Simple rule to remember, tension always stays the same from beginning to end.
3) Punch is elbow lead, not shoulder or hand - Don’t let the shoulder rise when doing the punch, it needs to be the solid base from which the elbow pushes from
4) Isolate the arm from the body- All to often you see individuals rising with their punch instead of created a solid stance for it.
5) Fist is to tight before the punch and after the punch - If you can imagine that you have a grape while starting the punch, and when you finish the motion you don’t crush it.
6) Over extending the punch and not keeping square - The body needs to remain square, our game is to able to use 2 hands equally.
7) Too much emphasis on the punch, it comes out naturally - If you try to hit hard, you'll end up using muscle and kill your own power.  You have to concentrate on the motion to generate the power.
Well, I hope this gets you started with the right mechanical motion of the punch.   As a student of Wing Chun, this is one area of basics that you need to master.
Written by Sifu Ed Cruz
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