The “Motherline”—In Chinese, the Jick Joong Seen or Jick Seen is an imaginary vertical line that passes through the middle/top of the head and down through the center of the body to the floor, forming an axis of rotation for the body. The Motherline does not change when a person pivots on their axis. However, if the person moves in any direction, the Motherline shifts accordingly. The Motherline, as opposed to the Centerline, is a Surface feature that runs through the center of the body. If you consider the body a cylinder and Spin it, the Centerline will, of course, move. The Motherline would not because that is what the Centerline would be doing.
The “Self-Centerline”—The Self-Centerline, is the vertical line that divides the body into two halves. When there is no opponent, the SelfCenterline runs down the head and body’s middle/front and back like a painted-on stripe. It can be used as a reference point for correct elbow and hand position during technique execution during forms practice. Specific block structures require that the elbow, wrist, or another part of the hand be on the Self-Centerline. In contrast, specific attack structures need the knuckles, palm heel, elbow point, or other areas to be central. When performing the Tan Sau motion in Siu Leem Tau, for example, the middle finger should point 45° inward toward the SelfCenterline from the origin of the action until it reaches that line and continues to follow it as the elbow is drawn in so that both the middle finger and the inner elbow end up on the SelfCenterline in the fully extended Tan Sau position.
In reality, the Self-Centerline arises from the Motherline and radiates outward from the axis of the body. When an opponent is present, the SelfCenterline is used as a reference point in the construction of Attack and Defense Pyramids and a primary target area. Most of the vital issues of the body fall somewhere on this line, front or back, so the Wing Chun fighter will usually focus his attack power on it. If you were to shoot an arrow into your opponent while aiming at the Self-Centerline, your attack would undoubtedly be more damaging than if the arrow penetrated any part of the body that was not on that line. Unless it were aimed at the Motherline from the outside and penetrated far enough to reach the vital organs the long way, the arrow would most likely not pass through any essential organ.
This is why the Self-Centerline must be carefully defended and why it is the primary target of the Wing Chun attack. Furthermore, when a punch lands off the Self-Centerline, the opponent can roll with the force of the blow using the Motherline as the pivotal point, effectively dissolving most of its impact. In contrast, a solid blow to a point on the Self-Centerline will be fully absorbed by the opponent because the pivotal moment is negated by the central focus of the punching power, leaving him no opportunity to “roll with the punch.”