As our depth of skill improves, we should feel our energy expand beyond the physical body. Practitioners, at very high levels, may feel their energy radiate out to fill the space of the universe. These concepts are very difficult to teach. External movements can easily be seen and students quickly mimic the movements of their teacher. But the real essence of T'ai Chi is hidden internally.
Words are inadequate to describe sensations that the student has never experienced.
Often analogies of everyday objects are helpful to introduce these concepts. Later, the student is more open to the actual physical experience. To help clarify the relationship of firmness and flowing, I will use the example of a tree and its relationship to wind. The health and strength of a tree depends on its roots. When the roots grow long and deep, they not only anchor the tree firmly to the ground, but they also draw more energy from the earth so that the top of the tree can grow taller and broader.