Chinese “Song” (松) is very hard to translate to English in Tai Chi scope. For simplicity, I will call it the state of not consciously using muscle strength.
Many Tai Chi classics and grandmasters emphasis the importance of Song. Why is so? I believe you have to go through this Song process to obtain the real Tai Chi power or Jin (劲). Without Song, you are still wondering outside of the Tai Chi gate. Song is a never-ending improvement process.
Here are some milestones of Tai Chi Song training process.
1.Song Jing (松静)
For your body to get into Song status, your mind needs to be free of any stress first. Jing (静) here means the mind stays calm and relaxed.
2.Song Chen (松沉)
“Chen” (沉) means sinking.
At this stage, you can re-direct incoming force down (sinking) to your feet and the ground will support you instead of your body muscle. Now you have developed the Tai Chi root. You are having Tai Chi Long Power (长劲).
3.Song Ling (松灵)
Ling (灵) means agile.
With further improvement, you now become more agile. You have strong root but you don’t need to rely on your root to support you. Your feet become free.
4.Song Tan (松弹)
Tan (弹) here means explosiveness, short power (短劲).
Tai Chi short power (短劲) also comes from Song. In classic it said “Strong power from extreme softness” (极柔软然后极坚刚).
Short elastic force can only come from Song state.
5.Song Wu (松无)
Wu (无) means nothing or disappeared.
At this stage, very high level of Song combined with very high level of sensing skill means your opponent can no longer feel, find, or try to fight you. Now you are getting close to “The others don’t know me but I know them clearly. That is the way to be an invisible hero.”