|Taekwondo Bible, Vol.2||
Kang-gi and Yu-gi, dependently upon their existence and non-exisitence of hardness and softness, can be the change of <Heo-Sil>.
¡°What are <Heo> and <Sil>? What has its appearance that has nothing intended in real is called <Heo>, while, on the contrary, what is filled with intention to do something regardless its appearance is called <Sil>.¡± ¡°As you know what your motion should be like and you know what points you should attack and what points you should not, then, riding the flow of <Heo> and <Sil>, you can succeed in your offense at the opponent.¡±(Ch. 29)
For an example, consider the case the opponent kicks you. Its defending technique is to concentrate your strength on the soft part behind his Kang-gi. On the other hand, if you avoid it stepping slightly aside, it is a Yu-gi as you follow his approaching toward you. This technique is to lead his offense to vainness with your <Heo> to his <Sil>. On the other, if your opponent respond to your <Sil> this way, you can also respond to his <Heo> with your <Heo>, making your kicking to light stepping, that is, to a faint kick, so that you can give him a strong attack with the next swift kick helped by the tide from your serial stepping. This is possible because ¡°<Heo> contains <Sil> while <Sil> contains <Heo>.¡±(Ch.29)
On the other hand, the technique of counter-kick of Taekwondo is to respond to the opponent's kick with your <Heo> thrusting his <Heo> that supports its <Sil> with your <Sil>. How can you do this? ¡°Seek some empty points in the opponent's motion. Then, go into it.¡±(Ch.38)