One of the first things a new martial arts student learns is how to stand, where to hold their hands to best protect themselves and how to position their feet to allow them to move easily and generate power for their strikes.
In southern Chinese kung fu right handed students will normally be taught to stand with their right foot forward, their hands extended in front of their body. This seems unusual to people who have trained in other fighting styles where a right hander is usually taught to stand left foot forward, hands close in to the body or head. The reason for the difference is that each of these stances are maximizing the use of two distinctly different ways of generating force. One is maximizing the potential momentum based power of strikes, the other is maximizing the thrust or structure based power.
The typical momentum based strike is typified by the modern boxer. They stand with their weak foot and hand forward (left hand for most) which they use to throw quick short punches to distract their opponent and set them up for a more powerful punch using their rear hand (right hand for most). When the boxer throws their powerful right hand punch they first pull their right shoulder back then swing it forwards as they twist their hips and extend their hand. As the arm reaches full extension they pop their rear (right) foot up onto its ball. (It should be noted that this is the way a modern boxer generates force, an old fashioned bare knuckle boxer actually used stances and strikes more similar to a southern Chinese kung fu stylist, because a bare knuckle boxing match was more similar to the type of combat a southern Chinese kung fu stylist is training for; i.e. no gloves).
Modern boxing with big heavy gloves in the video clip below:
A classic example of old style bare knuckle boxing in the video clip below:
The momentum of an object is the product of its mass and velocity or in layman’s terms how heavy it is and how fast it’s going. Pulling the shoulder back and then swinging it forwards with a twist of the hips allows the boxer to generate the maximum centripetal force from the hips and also give the longest distance for the fist to accelerate (increase speed) over. Popping the rear foot up onto the ball allows the hips to twist through fully and not reduce the momentum of the fist and glove. This type of strike makes perfect sense for a modern boxer because they wear soft gloves which absorb a lot of the shock of an impact. The gloves are heavy though, so if they can get moving fast enough the momentum will rock and daze the opponent (concussions in boxing are caused by the brain being shaken inside the skull rather than the sharp impact on the surface of the head).
Many other traditional martial arts styles use similar mechanics and force generation principles but from lower stances. Some people suggest that this is because the strikes were designed to penetrate through armour to damage the body of the opponent. A light of medium powered punch would not have much effect on an armoured opponent, so the martial artist learns to put everything they can into the one strike, using maximum acceleration over maximum distance.
Contrast this with southern Chinese kung fu. The martial artist is not wearing any gloves and opponents are generally unarmoured. How quickly the strike can reach the opponent becomes more important than the momentum it will strike with, as any strike with at least a moderate amount of force with the bare fist on an unarmoured body will cause pain and damage. Using the forearms to deflect incoming blows becomes more of a priority as well, as the southern stylist is also unarmoured and does not have thick gloves to absorb the impact of strikes to the head. For these reasons the hands are held extended in front of the body and another method of force generation becomes more advantageous.
Below is a video clip from the movie Ip Man displaying an excellent example of the extended guard, short striking and structural power generation:
This structural force is generated by short, sharp contractions of the connective tissue which links the body momentarily into a rigid object. In this way the martial artist can generate force by pushing of the ground. This is best achieved by striking with the powerful hand and leg forward, the opposite leg back and with both feet flat on the ground to maximize the instantaneous pushing force on impact.
For most self defense situations this kind structural/connective tissue power has distinct advantages as it is unlikely that either combatant will be wearing armour or heavy padded boxing gloves. The extended guard allows the martial artist to take the brunt of incoming attacks on the forearms rather than the body and head. The short distance the hands have to travel to the opponent, and the instantaneous nature of the force generation (a short sharp twitch of the connective tissue rather than momentum accumulated by accelerating the hand across some distance) allows the martial artist to throw a multitude of strikes in rapid succession at their opponent, maximizing the chance of at least some of them getting through. Once one strike is through, the following strikes will generally be easier to land as the opponent is momentarily distracted by the pain of the first strike.
This structural/connective tissue force can be compared to the thrusting of a spear, whereas momentum based force is more like the swinging of a hammer. Both are effective and each has tactical advantages depending on the situation. A good martial art will generally actually use a mixture of both these types of force. Southern Chinese kung fu is no exception, while it places great emphasis on structural force generation, it does also use momentum based force for striking as well, but this tends to be the exception rather than the rule. So for this reason the southern Chinese kung fu fighting stance normally places the strong arm and leg forwards.