When the opponent uses any Attack Pyramid, there is an opportunity to defend using the Centerline. As previously stated, all the Wing Chun fighter needs to do is correctly position his own Defense Pyramid to gain the Inside Centerline position. This should be done automatically, taking the shortest and most cost-effective route to Inside Centerline. For example, if your opponent throws a left punch from your right side (i.e., to the right of the Centerline), the most prudent and cost-effective defense would be to create an appropriate Defense Pyramid with either hand and wedge it between his punch and the Centerline, keeping him outside and to the right of that line and never allowing him to reach or pass it.
In other words, if he attacks you from either side of the Centerline, you should usually try to position your own Defense Pyramid before his hand reaches the line. Suppose the attack is directly on the Centerline. In that case, it can technically be deflected to either side. However, there will always be one option that is preferable because it gives you a better “set-up” in terms of Facing Advantage. If you had instead used either hand to block his left punch from the outside in, then carried it across the Centerline to end up on the left side of the line, you would have committed a tactical error known as Giu Sau, or “Forcing/Prying Hand.” Because you failed to recognize and take Advantage of the most convenient and expedient opening for correct Centerline defense, you had to force the opponent’s punch in.
In Chee Sau practice, the beginner frequently makes the Giu Sau error. For example, if his left hand made contact with his partner’s right hand or arm from the outside, and that partner attacked with that hand from anywhere to his right of the Centerline, the correct response would be to defend or change the line with footwork to gain Centerline Advantage. However, many beginners will try to push the attacking hand across the Centerline from the outside (from his left to right). The attacking hand will usually strike the left side of the face, with his left hand not only self-trapped due to its grip on the attacking hand but also assisting the opponent by amplifying the strike’s power.
In attacking mode, the Giu Sau error can also occur. For example, suppose you attempt to throw a proper punch from outside the Centerline, and your opponent correctly positions his Defense Pyramid between your point and the line. In that case, you will commit a Giu Sau error if you attempt to force your right arm across and strike again. The more prudent and cost-effective approach in this situation would have been to hit with the free left hand or to circularly whip the right hand to the inside or outside of the opponent’s Defense Pyramid, regaining the Inside Centerline position.
You could also change the line to reclaim Centerline Advantage with a new attack. The avoidance of Giu Sau becomes instinctive to the Wing Chun fighter after practice to develop the correct reactions. Half the battle is simply being aware of its existence. The other is to understand how to use the Centerline correctly.