Tai Chi places considerable emphasis on the use of vision while practicing. In the form, nearly each movement has a lead arm, and the lead arm is the one to watch. Because many of the movements of the form make use of the turning of the waist, this means that when we watch the lead hand, our head will also be turning in order to follow the moving of the hand. Watching the lead hand in this way also helps with the waist turning in itself.
Watching the lead hand however is only half of the story, there is also the other hand. As Master Wong would often specify, keep the other hand in the peripheral vision. In other words, by watching the lead hand, it does not mean we can loose track of the other hand. This is an easy fault to make simply because of the use of the language itself, if we ask our students, or our selves, to watch the lead hand, the statement itself will incline us to pay no attention to the other hand. I think that probably arises in particular from the word “watch.” The concept of watching does not include any idea of the peripheral vision.
In tai chi, then, we can not say, strictly speaking, that we are “watching” the lead hand because we are still also watching the peripheral vision. This is one of the subtle and interesting points of tai chi practice: tai chi is designed to train awareness of the peripheral vision: we are to learn the boundary of our peripheral vision, and be able to follow the movement happening in that sphere of vision. I do not know many arts or exercises that make use of peripheral vision.