Taichi theory is for the mind what Taichi practice is for the body.
Taichi "theory" is an extremely rich subject that cannot be simply summarized. In the same way that the movements and postures of Taichi Chuan must be practiced carefully and consistently over a long period of time before any real physical fluency or competency can be expected; as well the concepts and principles of Taichi theory must be studied and contemplated carefully and consistently over a long period of time before any genuine fluency or ease of understanding might be expected.
Of course like all things yin and yang, theory and practice are never truly separate. Physical experience is guided by intellectual process just as much as intellectual process is guided by physical experience. Throughout the very long history of Taichi theory in China, the dichotomy between theory and practice has been most commonly expressed through the terms of Martial (Wu 武) and Civil (Wen 文). The "martial" is classically understood as the outward expression or practice, while the "civil" is the inherent substance, principle or theory.
While the martial is transmitted through memorizing forms and postures and the physical act of moving and turning the body, the civil is transmitted through contemplating images and ideas and the mental act of knowing and feeling with the mind. Though the martial differs in expression a bit from person to person, the unity supplied through the civil is the same in all cases.
This understanding goes all the way back to the Yi Jing, or Classic of Change, from around 1000 BCE in the text for hexagram number 48, The Well: "The village changes but the well does not." About 500 years later it was echoed again by Laozi as: "All things vary in style and expression, yet each returns to its root the same. (Chapter 16)" And then Confucius shortly after with: "By nature people are close, by habit they become far. (Analects 17.2)" Fast forward another 2000 years to the ultra succint version found in the Taichi Chuan Treatise of Master Wang Zongyue from the 1500s: "Movement divides and stillness unites."
Taichi theory is not unique to Taichi Chuan. As a philosophical construct and organizational framework it is ubiquitous throughout the vast realm of East Asian cultural practices and life sciences. In Neoconfucian cosmology, for example, there is a Greater Taichi and a Lesser Taichi. The Greater Taichi is the universe itself: heaven, earth and the pulsing, circulating continuum generated through their harmonious intermingling (i.e. life). While the Lesser Taichi is any smaller universe within the larger one, such as a whole human being: mind, body and the pulsing, circulating continuum generated through their harmonious intermingling (i.e. life).
Taichi theory is not unique to China. Seeing the individual self as a microcosm of the universal whole is a fairly common feature in classical philosophies and mystical cosmologies from around the world. But the highly stylistic way this old understanding has been preserved and shepherded through centuries upon centuries of cultural upheaval and all manner of exploitation, misappropriation and persecution by being packaged up as a threefold cultivation system of martial art, healing exercise and spiritual awareness, is very much unique to the culture of China and spirit of the East.
From the very beginning in the Art of War by Sunzi, it was understood that strategy and diplomacy were far superior to open warfare and attacking enemies in terms of securing lasting victory. The only way to ensure the kind of "effortless victory without battle" that he advocated was through awareness and consideration over actions and urges ("always more formulations over no formulations"). There's a famous story about Zhuangzi, a beloved Daoist recluse who lived a couple hundred years after Sunzi, in which he confronts a whole host of the "best swordsmen in the land" with his long-winded poetic descriptions of his swords full of powerful concepts and ideas...and wins!
Again fast forward another 1500 years to the creation story of Taichi Chuan in which the Daoist recluse Zhang Sanfeng is sitting quietly in his mountain retreat studying the Yi Jing (Classic of Change) when he glimpses out the window a bird attacking a snake which keeps yielding and counterattacking until the bird flies away. From this confluence of thinking about the Yi Jing and witnessing the technique of the snake was born the idea of a martial art that focused on yielding and low serpentine movements that eventually came to be called "Taichi Fist" after "taichi," a special term from the Ten Wings of the Yi Jing used to describe the special dynamic of Yin and Yang when they are in mutual harmony.
Taichi theory is the secret to Taichi practice. Taichi practice is the secret to Taichi theory.