While visualizing might be important and useful for some forms of meditation, in tai chi physical sensation must take precedence to get the greatest benefit. The point of tai chi is to tune into and devolop physical sensation, and visualizing (another word for imagining) the effects you are trying to achieve is only a distraction, as it pulls the awareness away from the physical body and into the mind. Develop your awareness of the physical sensations of the practice, and sooner or later you will be able to feel energy moving through your body, and you won’t have to imagine it.