The following is an excerpt from the book, Wing Chun Kung Fu a Complete Guide, written by Dr. Joseph Wayne Smith. This book offers a good overview for those who are unfamiliar with Wing Chun.
“Classical Wing Chun kung-fu is typically defined by means of what is known as the center-line theory, which is the foundation of the system, determining both the fighting stance and the logic of combat. The center-line is taken to be an imaginary line that passes through the median axis of the body. It is on this axis that the most vulnerable organs of the body are located. The fighter therefore defends this area by keeping his elbows in the center-line or plane of the body in both attack and defense and seeks to attack the center-line of the opponent. Wing Chun attacks must typically be linear because of the requirement of keeping the elbows in the center-line; in addition, the shortest distance in space between two points (such as a knuckle and a nose), is a straight line, so linear attacks are much faster than circular roundhouse style swings. It is also more difficult to defend against straight-line attacks because they are direct, fast, and tend not to inform the opponent of the intent to attack.”
-Chap.1, pp. 3-4