Nowadays when we talk about tai chi, we have in mind of people doing elegant slow movements. Slow movement tai chi is great for health and structural building, a form of moving meditation if viewed from a chi and meditative perspective. Many contemporary practitioners who are interested in tai chi's martial art origin have abdominal punch taking and fa jing (pushing power) in mind (plus some "applications"). Agility being none-of-the-above. An interesting old video tells us that this was not necessarily the case when tai chi was practiced in the past.
The master in the video below is Dr. Chu Mingyi 褚民誼, a famous politician in the Republic era (and infamous, because he was later executed by KMT for treason) and a renowned student of Wu Jianquan the founder of Wu-style tai chi. Chu earned doctorate degree in medicine and pharmacology at the University of Brussels. It clearly showed that agility was the core concept of Chu's practice of tai chi. According to the video, Chu treated pushing hands as an effective form of tai chi conditioning exericse and that it required a partner who could deliver similar strength. Since finding such partner for regular practice was not easy, Chu (a rich man!) created two very interesting gadgets for his own training (see video).
Agility-focus in tai chi was also shown in another practice video (not shown here) by master Wu Dakui (吴大揆), elder son of Wu Gongyi (吳公儀) - father Wu showed his agility in his famous ring-fight with a White Crane master in Macau, son Wu was famous for his street level combat experience. Not too surprisingly, because in combat, one has to be agile and be able to deliver power at the same time. Power without agility gets one into trouble in real combat situations.
Anyway, it is refreshing to see that tai chi can also be practised in an agile manner, in addition to its slow movement chi-meditation and power generation training (of course and still other methods). Needless to say, one doesn't need to follow all approaches in practising tai chi. Each student should choose his own cup of tea, as Dr. Chu certainly did so decades ago.