Background – Summary/Review of Internal Power
As detailed in a previous two-part article series, internal power has two components – yin and yang aspects. The yang component is primarily core strength – learning to utilize core musculature to maximize power and efficiency of force output. It is the core musculature that flexes and stabilizes the spine, and it is core strength that is the mechanism of the classical saying “power is released from the spine.”
The yin component of internal power is improved inter- and intra-neuromuscular efficiency. This is the mind-body connection, and is the mechanism of the classical saying “the mind leads the qi, the body follows.” Taiji (tai chi) form is done slowly precisely because this is the most efficient way to exercise and improve neuromuscular efficiency.
The end physical result of correct practice is improved power and motor skill, both in performance and efficiency. Endurance to a taiji player is not the ability to “eat bitter” and withstand long periods of physical exertion – it is rather a function of efficiency of force output. The classical saying “four ounces deflects a thousand pounds” does not necessarily refer to total force output, it refers primarily to efficiency of force exertion. The force and power can be very large, but generated from the large, aerobic non-tiring core musculature without forfeiture of posture and balance. It is true that, in push-hands, one can utilize silk-reeling, leverage and “disappearing” to neutralize a large force with technique. But without the gong of internal power, this is a tenuous strategy at best. (To go a bit deeper, the employment of such skill on a non-willing partner is itself reliant on internal power, but we’ll look deeper at push-hands and silk-reeling in future posts.) To repeat the words of Grandmaster Feng Zhiqiang, whose internal power was legendary: “if you have the power, everything works.”
While all “internal” arts are characterized by the emphasis on developing and efficiently employing internal power, I would contend that balance of yin and yang is the meaning of taiji and therefore a fundamental defining characteristic of taijiquan, and that understanding or practicing the yin component without the yang, or visa-versa, is not taiji. Without understanding and skillful employment of yin and yang there is no real taiji skill.
Another way internal power is described traditionally is as an “elastic” force, to differentiate from the tense, stiff or rigid force resulting from contraction of the (easily exhausted) muscles of the external limbs. Let’s look a bit deeper at the mechanics of the elastic force from traditional and contemporary scientific perspectives.
A traditional view
The classic Song of Peng poem states that “the entire body is filled with a spring-like energy.” Quite importantly, the two lines prior to this verse are: “first sink the qi to the dantian, then hold the head as if suspended from above.” These two enigmatic lines simply refer to the physical mechanism of elongating the spine, or “pulling the spinal bow,” which stores energy throughout the entire core – just as potential energy is stored by pulling an elastic object like a rubber band.
While the core musculature is the strongest group of muscles in the body, the elastic force is potentially generated by the entire soft tissue/musculoskeletal system. It is the elastic force that is trained in the dynamic stretching movements of taiji and qigong, and a reason for my teacher’s constant admonition to “elongate” and to “make the movements bigger” – the greater the pull or “load,” the more energy stored and the more efficient one’s practice becomes in terms of developing and understanding energy and internal power. (Always, always, always, practice with the overriding principles of nurturing, relaxation and wuwei – of not using force to make a desired or imagined result happen quickly. I guarantee that if you attempt to incorporate this in your practice with brute force in hopes of obtaining quick results, you will: a) not learn taiji, and quite likely b) injure yourself. The potential force generated from the core musculature is relatively quite large, and once a rudimentary understanding of the mechanics is achieved it is not difficult to injure oneself through acute or chronic overuse – especially in the hips or lower back. This is one example of how the deeper and more efficient and intense your practice becomes, the less you need to, or even can, practice form).
Generating elastic energy is also the reason for the traditional teaching (often explained simply as one manifestation of yin/yang in the form) that if you want to go right (with power), you have to go left first – it is the storing of energy that occurs in a counter movement before a limb changes direction. The elastic force is also fundamentally trained in the jumping movements of the Chen style form – especially prevalent in pao cui, or the second routine. For several of these jumping movements, the traditional explanation I was exposed to was to “borrow” the energy from the ground, produced with an initial downwards countermovement, to spring upwards explosively and with agility.
The more I learn, the more I am amazed at the extent to which the old-time martial artists in China mastered human movement in terms of efficiency and power. Of course they did not have the explanatory lens we now have through contemporary science and so could only explain it in terms such as “storing qi.” That is not inaccurate, as qi is energy, but it does not explain much, and the general opacity of traditional language can easily lend itself to misunderstanding, or even outright fraudulent claims.
Under the age-old dictum “what is old is new again,” the “elastic” or “spring” force has reappeared over the past 20 years or so as a well-recognized mechanism for optimizing efficiency and power output in nearly every sport requiring explosive actions or endurance. In western sports science it is referred to as the “stretch shortening cycle,” or SSC. Training the SSC has been documented in the scientific literature to improve muscle fiber force and contraction velocity, and therefore power. 
A contemporary biomechanical perspective – the SSC
The SSC is defined technically as a pre-stretch or countermovement action composed of three parts. The first is an eccentric contraction (elongation) of muscle in which energy is stored through the elastic properties of soft tissue (primarily thought to be in the tendons, though the potential contribution of other soft tissue, such as the fascia, has not yet been fully studied), followed by a transitional period called the amortization phase. The third phase is a concentric contraction of the muscle fiber, which is the body’s natural response to the elongation. The stored elastic energy produces a greater force during the concentric contraction than can be generated by an isolated concentric muscle contraction.
Though the SSC is a well-recognized mechanism for both energy conservation and increased propulsive force, the exact biomechanical mechanisms are still not known or agreed upon. It is generally believed that two of the more plausible explanations are elastic energy through tendon recoil, as well as an increase in the active state during the eccentric and amortization phases, which allows the formation of more cross bridges before the concentric contraction.  You can search ‘stretch shortening cycle mechanisms’ to research aditional theories or hypotheses. One of the more interesting hypotheses for taiji and qigong is the possible role of the fascia, which is a viscoelastic matrix that envelops muscles, bones and organs and forms a continuous network throughout the body. The fascia is a piezoelectric material that changes mechanical force into electric energy (qi? :-), and is a connective signaling network throughout the entire body. During a muscular contraction the fascial expansions can transmit the tension generated by the muscle to neighboring areas, stimulating the proprioceptors in that area – part of a mechanistic hypothesis of the SSC known as the neurophysical model.  (As a side note, the piezoelectric and connective signalling properties of the fascia is also a proposed mechanism for acupuncture.)
The classic example of the SSC is jumping. Compared to a squat jump, you can jump higher with a running start, or from a static position by bending first (called a countermovement jump), because elastic energy is stored in the soft tissue by the initial pre-stretch or countermovement. Note that the length of the transitional phase is important – if it is too long, the elastic energy is dissipated as heat and therefore no energy boost remains available for the concentric action. This is rather obvious – for example, if you stop completely after a running start, or after bending down, the jump just becomes a squat jump. (This is also a significant reason why dynamic stretching is considered more effective than static stretching for athletic performance.) To return to taiji briefly, I believe this is one important reason why traditional teaching emphasizes constant, fluid motion, and never completely stopping. If your movement is “square,” stiff or halted, as opposed to “circular” and fluid, you lose the elastic energy stored in the yin phase. Rather, in taiji movement you store and release, expand (elongate) and contract, move left and then right, up and then down, etc., in a smooth, fluid and continual yin/yang cycle – thus training the body to maximize energy storage and potential force output in any direction.
The SSC is the mechanism of energy storage in the wind up of pitchers, the cocking of the bat and step forward by batters before swinging, the stroke of a tennis player, the throwing motion of a quarterback – nearly every human motion involves a countermovement action, and maximizing the athlete’s ability to employ elastic force stored in countermovement is a big area of focus in sports training today.
In boxing, extending the forward (jab) arm to touch the opponent before throwing the cross is called “posting.” This motion has two purposes: gauging distance, and storing energy through the SSC to make the cross more powerful. Watch 0:51-2:10 of this video explaining the mechanics of Joe Lewis’s famous right cross – and note the explanation of the movement as maximizing force output through the mechanics of the SSC:
Now try the movement yourself. You will (or should) recognize the feeling of that energy as being present in the preparatory or yin phase of every movement of the taiji form and dynamic qigong exercises. The SSC is the physical mechanism of taiji’s “elastic force.”
The unique physical mechanism of the internal martial arts, I believe, is the utilization of the SSC through flexion of the spine. It is this biomechanical motion that allows maximum energy to be stored without countermovement of the limbs, and therefore, significantly, without telegraphing intent to an opponent. That this biomechanical motion of storing energy is not apparent to an unknowing observer is the reason why so much mystery, misunderstanding and imaginary explanations were not uncommon throughout taiji history. It is also the flexion of the spine that makes the generation of very short range or “inch force” possible, in any direction, with or without the aid of the waist turning. This power is not known to most athletes or sports trainers. It is not a natural ability and is the “power that has to be learned” from a knowing and willing teacher.
I can testify to one thing – I can hit a golf ball as far with a seven-iron now (in my mid fifties) as I could with a wood before I started taiji (in my mid twenties). Still can’t hit it straight, but that’s another thing.
All of the above is really addressing the storing of power in the body – the yin phase of any taiji movement. How that power is most efficiently released through the kinetic chain is silk-reeling. More about that in future posts.
 Malisoux L, Francaux M, Nielens H, Theisen D. Stretch-shortening cycle exercises: an effective training paradigm to enhance power output of human single muscle fibers. J Appl Physiol (1985). 2006 Mar;100(3):771-9. Epub 2005 Dec 1.
 Turner, Anthony & Jeffreys, Ian. (2010). The Stretch-Shortening Cycle: Proposed Mechanisms and Methods for Enhancement. Strength & Conditioning Journal. 32. 87-99. 10.1519/SSC.0b013e3181e928f9.
 Walker, Owen. (2016, Jan 23). Stretch Shortening Cycle. Retrieved from https://www.scienceforsport.com/stretch-shortening-cycle/.