External and InternalThere are many definitions of external and internal martial arts, most of which I find unsatisfactory. I prefer a more pragmatic definition:
An external art is one where the power generation mechanism is easily visible to the naked eye. For example, southern arts like Hung Kuen or Choy Lay Fat, northern Shaolin like Cha Quan etc., utilize primarily the body torso rotation around a vertical axis to generate power. The detail of how it is done is somewhat unique to each art, and it makes each art distinctive.
The power generation mechanism of the internal arts, on the other hand, is much harder to see by an inexperienced observer, especially when loose clothing is worn. It is much more subtle and the movements are much smaller (hence short power emission (i.e. at close distances) is very common). The action relies on the subtle movement along the spine, bending back and forth in different directions.
How to attain internal power每一家都有自己的獨特功法：太極站無極樁，形意用三體式，意拳站樁換勁，心意蹲猴，八卦走圈換掌，南派洪拳練扎馬，蔡李佛首練五輪馬，而詠春最重小念頭。
Each style has its own distinctive way to train the body: taiji uses the wuji stance, yiquan has different stances for different purposes, xingyi uses the santi posture, Shanxi xinyi uses the squatting monkey posture, bagua trains by walking in a circle, etc. In southern arts, Hung Kuen is known for its strong horse training, Choy Lay Fat also trains the horse but with more nimble footwork, and Wing Chun uses the Siu Nim Tau.
The TheoryDisclaimer: knowing and understanding the theory does absolutely nothing for improving your kung fu. Only countless hours of mindful practice will do that.
I don't have a good drawing program, so I am just going to wave my hands and talk my way through it without using pictures. We are going to run a series of thought experiments, so put on the safety goggles and fasten your seat belts.
Imagine a vertical wall with a horizontal rope fastened on it, just like in standard high school or college physics texts. If you pull the rope horizontally away from the wall, you can make the rope taut. If you hang a weight on the rope, you can make the rope taut again by increasing the tension (i.e. pull harder in the same direction as previously). Technically it's called increasing the horizontal force.
Now imagine, if instead of a simple rope, we have a segmented rope similar to those segmented snake toys you can sometimes find in a toy store (those which have the snake head wiggle when you wiggle the tail), or like a multi-sectional Chinese steel whip. Obviously the same thing works, i.e. you can support a weight (vertical force) by suitably increasing the tension (horizontal force). So far, so good.
Now imagine instead of having the sections being a uniform length, they come in different lengths, so some are longer than others. No problem, it gets more complicated but everything still works similarly as before.
Next, make these segments different shapes, so besides being different in length, they are also different in shape, so some may be thicker or thinner, others may be triangular, polygonal, or just plain irregular in shape. Again everything still works as before as long as they are attached together, even if the attachment points are at odd angles.
Suppose the individual segments are piecewise attached using something like heat shrink tubing, so the joints are flexible while the segments are rigid, and everything is hanging together as before.
Now for the critical step, rotate the picture 90 degrees so the vertical wall becomes the horizontal floor, the vertical "weight" now becomes a horizontal force, and the "rope" is now standing vertically. So long as tension is maintained (by pulling vertically up) the rope remains taut when a horizontal force is applied.
If you imagine a little harder, you can see the non-uniform, multi-segmented rope is very similar to the human spine/skeleton. The ground attachment is not a single point but a complicated collection of bones in the foot. Proper placement of the foot (aka foot bow) and tibia angle results in a suction like action which helps to stabilize the attachment point and help direct the incoming force into the ground.
Some object to TST's demos as being simple physics tricks (which it is), and useless, which it is not (because of Newton's 3rd Law, the force his body can generate is equal to that of the pusher, hence he is capable of delivering a very powerful horizontal force) and hence infer that he is a charlatan. I don't agree with that assessment. It is not easy to control your body to that extent.
Insight #1: if you create tension (a very misleading word) in the physics sense in the vertical direction by imagining a straightening of the spine (the spine really cannot be straightened too much, but you can have the intent to try straightening it), you can withstand a horizontal push (think TST's demos). The tension is created by keeping the head suspended (蹬頭dung tau), flexing the spine in the right direction (挺腰ting yiu), and dropping weight into the knees (落馬lok ma). By minimizing non-essential muscular tension (there's that word again), the tension (in the physics sense) can be distributed more evenly along the spine/skeleton as opposed to concentrating in a few places (blocking the qi) where it can be easily overwhelmed by the external force. It is a very good thing that the spine is S shaped, because if it were straight, the vertebrae will be stacked directly on top of each other and it is much harder to get the horizontal forces canceled between the links. As it is, at each joint, because of the difference in alignment, the horizontal force gets resolved into longitudinal and transverse components. The combination of all the joints tend to counterbalance the incoming horizontal force (up to a limit). There's a whole lot of complicated things going on along the spine, and all of the joints together works like a bow.
Insight #2: the skeleton coupled with relaxed muscle usage creates a frame. Force can be issued and received with minimal movement in the joints (minimal flexing of the frame). This is known as whole body force(整勁), which is a more efficient use of the body's resources than localized muscles.
This is a first approximation to what I think is happening. I have left out a lot of elements (e.g. the use of the dantian) which may contribute to how things are really done, but it helps to have a mental picture which, while not complete, may offer a rough sketch as to what is going on. I have been thinking along the lines of this theory for a few years now and I haven't seen anything which contradicts or pokes holes in it yet. If anyone can think of reasons why it is wrong or where it should be amended please let me know. It is similar to Mike Sigman's basketball post model but whereas his example is rigid where all the internal dynamics are hidden, mine is more explicit and perhaps, easier to understand.