According to Huang Sheng-Shyan, the difference between taiji and other martial arts, is that taiji can ultimately develop jeijin (receiving energy), where yielding, neutralizing and discharging, all happen simultaneously. There is hardly any physical movement, and no mental intention at all, everything happens spontaneously and naturally.
The practitioner is in a state of absolute central equilibrium, the posture is totally connected and relaxed with the feet deeply rooted. The mind is calm and as still as a mountain. By being totally connected, connected and relaxed the body become an empty void. When an external force contacts, the body does not resist it, the force just passes through until it hits the ground and rebounds back throwing the opponent. Similar to pile driving during construction work, the deeper the pile is driven into the earth, the higher the hammer rebounds.
Achieving jeijin (receiving energy) indicates attaining shenming (taiji enlightment), at which point (sparring) techniques becomes irrelevant.
Taijiquan Wuwei, Kee-Jin Wee Oct 2003