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 of Li Yanxi
Nei Jing wikipedia.org