William C.C . Chen has a good text on his homepage on the Tai Chi foot called The Mechanics of the Three Nails. The three nails that Mr. Chen is speaking about is on the inside of the feet and located on the big toe, inside of the ball of he foot and on the heel.
It’s a very good article and has more wisdom that many would realize without really test what is in it. I do agree on that the foot should not be totally relaxed or too soft. However, I would personally focus even more on activity. In Taijiquan, the feet should be alive, moving and take an active part of the movement. Mr Chen says that this as well but in another way.
But it’s hard to grasp the activity without practicing and feeling the dynamic movement. One simple exercise that I would propose is to stand naturally in either a “wuji” position or straight up, feet shoulder width and with the inside of the feet parallell to each other. Or you can stand in a small bow stance or in a small oblique/sideways horse stance. It doesn’t really matter what stance you choose, but keep it small and natural. First relax the feet as much as you can. Feel it spread out like a fan below your weight. Then try to grasp the floor with your feet. The ball of the foot must stay firm on the ground. Try to grasp the floor using the space between the toes and the ball of the foot, and also with the space between the heel and the ball. Now, shift your weight while coordinating this squeezing and relaxing of the foot. Do this for a while, just feeling what’s happening. Now, keep the three nails firm on the ground. Then press or squeeze down your little toe to move the weight from one foot to another.
The important point is to really activate the foot, not keeping it dead or just as something supporting the weight. Practice this while doing your form or practicing push hands with a friend. This will give your whole body a certain alertness and aliveness. You will understand the difference when you get hang of it.