While moving chi is the purpose of learning Tai Chi no matter what you’ll ultimately do with it, focusing on the form will help build defensive moves. To get a form and its moves down pat, practicing fast can alert the student to areas that need attention. Since the form itself, which defines the position of the body (arms, waist, legs, etc.) is the foundation for defending yourself, that is where your practice must lie.
Certainly moving from within and allowing the chi to define your guard ultimately yields the best defense. But until each move is mastered, that aspect won’t help much. So focus on the form first. Practice being flexible and balanced with each move. As I stated above, make sure you are centered at the dan tien so that when a foot is lifted during a move, you are still balanced and firm within your stance. Practice breathing in when pulling away (inward movement) and breathing out when pushing outward. The flow of the breath during each move is very important for defensive purposes. Never, never, never hold your breath or disrupt your normal in/out breathing during practice. You can exaggerate your breathing to get a better feel for how it flows with movement, but don’t maintain anything but normal breath during practice.
Doing Standing Meditation (SM) on one leg is an excellent way to gain greater balance and the centering of your body. Get into the normal position with back straight, knees bent and arms forming a circle like holding a ball (see my previous article describing SM in detail).
Once you are centered at the dan tien and feel chi flowing from one hand to the other, put all of you weight on one leg and regain balance.
Then lift the other leg by bending the knee and bringing it up a comfortable distance, preferably to the waist. Stay in that position for as long as is comfortable. Then switch legs and repeat.