In order to compare and discuss these two methods I will call the first momentum techniques and the second balgyeong or impulse techniques.
Since the inception of the Theory of Power, ITF Taekwon-Do techniques focussed on momentum. Speaking of “momentum” I'm really speaking about Force, in the classical mechanics sense, where Force is the product of mass and acceleration: F = m x a. The idea is to “accelerate as much body mass as possible in the direction of the technique,” as I explained in a previous post on power generation in Taekwon-Do. Almost every technique pushes / breaks / drives through the target like a locomotive through an obstacle. Probably the place where one can truly see this in action is in power-breaking. Look, for instance, at the power-breaking demonstration in the video below. Notice how the techniques drive through the target, continuing past the moment of impact—fulfilling the martial art saying to “aim behind the [surface of the[ target” or “strike through the target.”
The effectiveness of these techniques are undeniable.
Balgyeong techniques work differently, although the idea of using momentum is still apt. In balgyeong the momentum is accelerated and then transferred into the target, instead of driven through the target. The preparatory position or “pull back” that is needed for greater acceleration (the greater the distance, the more time there is for acceleration) is usually far less in balgyeong techniques. Acceleration in balgyeong is achieved through kinetic chaining instead. Glen Levy shows off his balgyeong / fajin strikes in the video below.
Finding good examples online of balgyeong / fajin power-breaking is very difficult, in part because it is very difficult to break things with balgyeong. One might therefore be tempted to think that since balgyeong cannot break bricks as easily as momentum techniques can, that balgyeong is therefore inferior. This is a bad assumption, in part because balgyeong is not meant for breaking objects. To break something you have to push against an object with continued pressure until its structural integrity gives way and it snaps. Balgyeong does not work like that. A balgyeong technique transfer energy into the target, not through the target.
|"WTF Chest Protector"|
You can practise these two methods on a punching / kicking bag. Some techniques will push the bag, so that it swings back forcefully. These are momentum techniques. Other techniques, if done with balgyeong, will not make the bag swing much; instead the bag will shudder as the force is transferred as an impulse into the bag.
So how do these two methods translate when applied on a human being? Well, the momentum technique will usually push the opponent back. If you do the technique with great acceleration, then the technique may actually break the bones of the opponent, for instance breaking his ribs or other bones—something I've experienced personally. A balgyeong technique, on the other hand, will not push the opponent back that much and is unlikely to break bones. Rather, the force goes into the opponent as an impulse of energy causing internal trauma. Instead of breaking bones, a balgyeong technique could harm the internal organs, for instance a blow to the chest could possibly cause cardiac arrest. Balgyeong techniques are also more likely to shock the nervous system; that is why so many people whom have experienced a balgyeong strike say that they suddenly feel weak, as if all their energy has been drained from them.
The question which is better, momentum techniques or balgyeong techniques, being hit with a crowbar or with a ball-on-chain, is a bad question. It's comparing apples with oranges. The real question should be: “What do you hope to achieve with the technique?” Momentum techniques and balgyeong / impulse techniques are different methods of transferring energy, with different results. Depending on the desired result, one would choose the appropriate method. Regarding the question if balgyeong is a valuable contribution to the style I would answer “yes.” It gives the ITF Taekwon-Do practitioner more options and a greater arsenal of techniques. While momentum techniques were very effective within Taekwon-Do's original context as a military martial art, I think balgyeong techniques bring value to Taekwon-Do in its new context as a system for civilian defence. (Read more about "civilian defence systems" here.)