“Wang Zhengnan’s epitaph” (王征南墓志铭) written by Huang Zongxi (黄宗羲) in around 1670, is the first document differentiated Chinese Internal and External martial art styles.
The definition for Internal art is “Using Stillness to control Movement” (以静制动) in contrary to External art’s “Focus on attacking” (主于搏人). Thus, the training approach is also different.
External styles emphasis on speed and strength training while Internal styles focus on mind and sensitivity training.
Chinese word Jing (静) can be translated as Stillness, Calm, Static, Tranquillity etc. The most important thing is to control your mind to reach Tranquillity status then you can develop better sensitivity thus better/faster reaction. It also brings a great health benefit in dealing with stress related issues in current society.
Sitting and Standing Meditation are commonly used in Internal style training such as Tai Chi to develop mind control. It applies to form training as well. One of the ten guiding principles from Yang style Tai Chi’s Grandmaster Yang Chengfu is “Seek stillness in movement (动中求静)”. Being soft and relaxed in form practice is important to maintain the tranquillity of the mind.
From Jing (静) to Ling (灵), to reach the status as described in Wang Zhongyue “Taijiquan Treatise”: “The others don’t know me but I know them clearly. That is the way to be an invisible hero.” (人不知我，我独知人。英雄所向无敌。) This is the training path we take in our school.