To be relaxed means to release tension, but not let go of substance. There is a quality in-between stiff and loose which is stable, yet flexible, that has fullness without being rigid, that is calm in motion yet conveys a vigorous presence. For lack of an equivalent English word, I refer to this concept as flowing within firmness, firmness within flowing. Flowing and firmness do not gain support from a rigid skeletal posture or strength from muscular tension. Rather, their integrity comes from expansion.
Expansion is the ability to spread out in all directions.
This is the key to relaxing without collapsing. The ability to expand depends upon a foundation of good posture. The principles of T'ai Chi Ch'uan outline the basics for maintaining a posture that functions in the most efficient way within our environment. By aligning the body to be in harmony with the forces of nature, we use the earth for centering and support and the heavens as a counterpoint for balance. T'ai Chi, we need to literally feel as if our feet are reaching deep into the ground so that we are firmly anchored.