Light, agile, and alive, seek Dong Jin (understanding Jin);
Yin and Yang cooperate mutually without the fault of stagnation;
If (you) acquire (the trick), four ounces neutralizes one thousand pounds;
Expand and close, stimulate the “drum,” the center will be steady.
“Alive” here means alert and active. In practice you must pay close attention to your partner. In time, you can interpret his intentions from the slightest of motions (i.e., understanding Jin). Where your parter is heavy your are light. When one part of you is light, another part of you is heavy. You and your partner continually follow one another, never resisting, never separating. In this way the motion will continue to flow. In time you will acquire the knack of being light enough to avoid your partner’s attack, and substantial and controlled enough to deflect or attack him. Your postures alternatively open and close with the circumstances. The drum is the abdomen, in the area of the Lower Dan Tian. You stimulate the Qi centered there with sound, attention, breathing and movement. This strengthens the Qi and exercises the control of it. When you attention and actions are thus centered on the Lower Dan Tian, your stance will be stable, and your mind calm and clear.
Reference: Tai Chi Secrets of the Ancient Masters Selected Readings with Commentary by Dr. Yang, Jwing-Ming