When you practice the spirit should remain inside, not going out.

Even during combat keep the spirit inside and calm. 
Internally I watch you like a cat. I watch you closely to see what is your next move.

Otherwise, if your spirit is exposed your whole body will be tense.

T.T. Liang
quoted from, “Quoted from Lessons with Master Liang: T’ai-Chi, Philosophy, and Life.” Quoted by Scott Rodell

Found HERE