Wing Chun is a traditional Chinese martial passed on from Ip Man, one of Ip Man's most famous students was Bruce Lee who trained in Wing Chun before going onto create Jeet Kune Do.
Wing Chun is known for trapping and for its fast strikes.
Wing Chun punches are delivered using Fa Jing, a short burst of energy in which the arm is momentarily tensed at the conclusion of the strike. This concept is similar to impact loading in engineering.
Wing Chun favours the vertical punch for several reasons:
Directness. The punch is not "loaded" by pulling the elbow behind the body. The punch travels straight towards the target from the guard position (hands are held in front of the chest).
Protection. The elbow is kept low to cover the front midsection of the body. It is more difficult for an opponent to execute an elbow lock/break when the elbow occupies this position. This aids in generating power by use of the entire body structure rather than only the arm to strike. Also with the elbow down, it offers less opening for the body to be attacked while the forearm and punch intercept space towards the head and upper body.
Strength and Impact. Wing Chun practitioners believe that because the elbow is behind the fist during the strike, it is thereby supported by the strength of the entire body rather than just a swinging fist, and therefore has more impact. A common analogy is a baseball bat being swung at someone's head (a round-house punch), as opposed to the butt end of the bat being thrust forward into the opponent's face (wing chun punch), which would cause far more damage than a glancing hit and is not as easy to evade. Many skilled practitioners pride themselves on being able to generate "short power" or large amount of power in a short space. A common demonstration of this is the "one-inch punch", a punch that starts only an inch away from the target yet delivers an explosive amount of force.
Alignment & Structure. Because of Wing Chun's usage of stance, the vertical punch is thus more suitable. The limb directly in front of the chest, elbow down, vertical nature of the punch allows a practitioner to absorb the rebound of the punch by directing it through the elbows and into the stance. This is a desirable trait to a Wing Chun practitioner because it promotes use of the entire body structure to generate power. Whereas, the rebound of a horizontal punch uses only the arm to strike. In this elbow-out position the hinge-structure directs force outwards along the limb producing torque in the puncher's body.
The Wing Chun practitioner develops reflexes within the searching of unsecured defenses through use of sensitivity. Training through Chi Sao with a training partner, one practices the trapping of hands. When an opponent is "trapped", he or she becomes immobile.
"Greet what arrives, escort what leaves and rush upon loss of contact"- Ip Man