Taijiquan was developed as a martial art many centuries ago. In all likelihood it was practised by vigorous young men who worked as farmers, that is, by men already physically strong with good fiery spirits. These men would have been taught relaxation and turning so that their strong bodies could borrow and return energy. Hours and hours of pushing hands would have taught them to listen and stick to their opponent, and hence yield/attack was born (can't have one without the other). Nowadays, Taiji is practised by middle class office workers: well educated, well meaning folk with weak slack bodies and slumbering spirits, full of neurotic tension and overactive minds. Such students require a different approach to those of old: focused meditation to quieten the mind, vigorous labor to strengthen the body, and some form of fighting to tone and hone the spirit. Otherwise Taiji has no authenticity, no heart, and the wrong sort of mind.