“The Inner Force is concealed until it is released.”
Perhaps the most difficult aspect of the martial arts to master is to learn how to conserve energy and to relax while under stress. Ki cannot be released if the body is stricken with inner tension and fear. Tension is also the greatest consumer of energy; even when the body is not in motion, one can become exhausted simply by being tense. When you are calm, you are free from hesitation. These are essential elements of conserving energy.
Of course, we must not overexert ourselves while training either. When the body is fatigued, errors are committed and constant repetition only makes bad habits.