The Mohe zhiguan is a manual, and its subject is the pair of practices its title names. Zhi is the stilling of mental movement, the settling of attention; guan is the discernment that operates once things are still. Zhiyi treats them as inseparable, two wings rather than two stages, and the book works through the preparations, the postures, the obstacles, the illnesses that arise, and the states that can be mistaken for attainment.
Underneath the practical instruction lies the doctrine the school is known for. The threefold truth holds that every phenomenon is at once empty of independent existence, provisionally real as a conventional designation, and, in the very fact that both statements hold together, the middle. These are not three levels to be climbed but three aspects of a single perception. From this follows the formula of three thousand realms in a single moment of thought: every possible mode of existence, from the hells to buddhahood, is present in whatever thought is occurring now, so nothing has to be acquired from elsewhere.
The consequences are practical rather than speculative. If buddhahood is not somewhere else, then meditation is not a journey but a recognition, and there is no realm of experience, however degraded, that lies outside practice. Later Tiantai thinkers drew from this the conclusion that even a buddha retains the potential for evil, a position that startled other schools and that follows directly from taking the doctrine seriously.