Discriminate Insubstantial and Substantial (分虛實) from The Ten Important Points of Tai Chi Chuan (太極拳十要)

分虛實

太極拳術以分虛實為第一義，如全身皆坐在右腿，則右腿為實，左腿為虛﹔全身皆坐在左腿，則左腿為實，右腿為虛。虛實能分，而后轉動輕靈，毫不費力﹔如不能分，則邁步重滯，自立不穩，而易為人所牽動。

楊澄甫

"The discrimination of insubstantial and substantial is the most important content in Taijiquan. If the entire body is all sitting on the right leg, then the right leg is substantial and the left leg is insubstantial. If the entire body is all sitting on the left leg, then the left leg is substantial and the right leg is insubstantial. When the insubstantial and substantial can be discriminated, then the turning and movement can be light and agile, without wasting too much power. If (they) cannot be discriminated, then the stepping is heavy and stagnant, the self stance is not steady and can be pulled and moved easily by the opponent."

By Yáng Chéngfǔ (楊澄甫), based on the translation by by Yang Jwing Ming (楊俊敏) presented in the book Tai Chi Secrets of the Yang Style (太極拳楊氏先哲秘要) and various translations on the internet