‘In the fully energized state, “every hair is fully alert.” The state of relaxed arousal is what is meant by the chinese term “sung.” This is not the drowsy torpor before sleep. It is the release of tension that saps our strength – so that we become alert, clearheaded and full of vigor. Your head is uplifted and your eyes open, while letting go of physical tension in your muscles and organs.’
(The Way of Power by Master Lam Kam Chuen p. 134)

Cheng Man Ch’ing on Sung

I have been practicing Tai-Chi Chuan for over fifty years. Only two years ago that I started to understand the word “relax”. I remember my Tai-Chi Chuan teacher Yang Cheng-Fu who did not like to talk much and he used to sit all day without saying a word if no one asked him questions. However, in our T’ai-chi class he would tell us to “relax” repeatedly. Sometimes it seemed like he would say the word hundreds of times during the practice so that the word could fill up my ears. Strangely enough he also said that if he did not tell me of this word that I would not be able to learn T’ai-chi in three life-times (meaning never). I doubted his words then. Now that I think back, I truly believe that if he did not keep reminding me of the word “relax”, I doubt if I could have learned T’ai-chi Chuan in six life-times.

What is the meaning of “relax” in T’ai-chi? Here is an example to help you understand the word. When we go visit a Buddhist temple we usually see a statue of Me-Lo Buddha. The one who has a big rounded stomach with a big smile on his face. He carries a large bag on his shoulder. On top of this statue we see a motto: “Sit with a bag. Walk with a bag. It would be such a relief to drop the bag.” What does all this mean? To me, a person himself or herself is a bag. Everything he or she owns is baggage, including one’s children, family, position and wealth. It is difficult to drop any of one’s baggage, especially the “self” bag.

T’ai-chi Chuan is difficult to learn. To relax in practicing T’ai-chi Chuan is the most difficult phase to go through. To relax a person’s mind is the most significant obstacle to overcome in practicing T’ai-Chi. It takes a great effort to train and exercise one’s mind to relax (or drop one’s “self” bag).

The way to nurture the force of ‘Nei Jing’
Within martial arts, the key to unlock and nurture stronger inner energy of ‘Nei Jing’ is through practising ‘song’ (Traditional Chinese: 鬆 ). The term ‘song’ can function as a verb which means to keep one’s mind and body loose resilient and expanding like the consistency of cotton or clouds or relaxed yet concentrated like the sharp alertness of cats immediately before attack. The term can also be used as an adjective which has the same meaning as described above. The greater the extent one can achieve ‘song’ and minize the use of ‘Li’, the greater the release of ‘Nei Jing’ force.

Practising ‘song’ is part of kung fu training process. It occurs when one keeps reminding oneself to ‘song’ thoroughly and refrain from the ‘Li’ force because the energy of ‘Nei Jing’ will be locked and blocked whenever the force ‘Li’ is applied. So, ‘Nei Jing’ and ‘Li’ are said to be mutually exclusive.

Therefore, the Tai Ji Quan master Yang Chengfu used the concept of‘song’ as a benchmark in his daily teaching. It was his daily routine to keep reminding his disciples to ‘song’ thoroughly more than 10 times when he inspected them.

Nei Jing wikipedia.org

Nei Jing of Li Yanxi

Sung of Wu style taiji Master Zhu Datong