A key principle/technique in Wing Tsun/Chun and all derivations, "bong sao" translates roughly as "Wing Arm." There is much disagreement about the correct position of a bong sao, and what its exact purpose is. This wu will reflect the position of Leung Ting Wing Tsun, as this is my school.
The bong sao is both a defensive and, occasionally, offensive maneuver. Statically, it is achieved by holding the upper arm parallel to the ground and at a right angle to the body. The shoulder should be settled. The forearm, which contacts the opponent during the application of a bong sao, is held at a 45 degree angle from the line of the upper arm, in three dimensions. To better visualize this, imagine a cube and draw a line inside the cube from one corner to the corner farthest from it. This is a spatial diagonal, employed to create the most surface area and, thus, the best defensive situation.
The feeling of bong sao should be a sensation of springy expansion. As always, it is the transition between principles that is of most importance in Wing Tsun, and thus the static posture of bong sao is seen to be essentially meaningless. It is a point on a roadmap, not even a rest area . . . One of the core movements from chi sao (sticking hands), it is used primarily as a defense against attacks which cross the centerline. Bong sao can also be used as a close-range strike in some rare scenarios, with application similar to lan sao.
Many people claim that bong sao is found only in Wing Tsun/Chun. However, it appears as the principle of "bong" in Tai Chi, as well. I myself have seen it applied once by a high-level practitioner of Chen Style Tai Chi. Also, in Si-Gung Kieth Kernspecht's wonderful book, On Single Combat, there is a discussion on whether bong sao is a natural reaction. The conclusion: it is. If you watch a boxing match, you may very well catch multiple glimpses of this principle (however correctly or incorrectly applied) during the fight. | 
