"use mind, not force""use mind, not force"
This important teaching must be followed at all times.
To explore its meaning more thoroughly, it is useful to consider the two
main characters. 
yi(4) means idea, meaning, intent. The character is made
up of 2 radicals. The top piece 
is a modification
of the radical for word or speak and means
sound. The bottom piece 
is a pictograph of
a heart. In Chinese culture as well as many other traditions, the heart
and the mind are not separate. This radical usually conveys feelings, moods,
emotions. It means heart, mind, center. The sound of the heart is the mind intent or idea.
The other character is 
li(4). This is a pictograph of a tendon, and
means strength, power, force.
In TaiJi Quan practice, we endeavor to to make the qi follow the direction of the mind. This idea is one of great significance. This is the essence of the internal school, of the soft overcoming the hard. In external martial arts, the emphasis is on forcing the muscles to bind the bones. The ensuing tension and stiffness is antitethetic to the practice of the TaiJi Quan, as taught by our teachers. The practice of song (relax) is quite different than the experience of li (force.)