The essential movement principle of Taijiquan is based on relaxation (song). In life people learn to use strength and tighten their muscles habitually. What is required in Taijiquan is, when you do an action, to execute the action to its optimum place, and then to make sure that it is not an isolated action but part of a whole moving system. At the same time further increase the idea of song through mindfulness. Song is unquantifiable and its quality is different with practice. It is important to continually improve the quality of song.
The arms, for example - as soon as the arms move the shoulders should relax and the elbows sink down. The arms also expand outwards. At a certain point of accomplishment, the arms become an integral whole with the spine, and actions of the shoulders and elbows become connected to the spine and controlled by the spine. After an extended time of practice, looseness of the arms is achieved.
The waist/crotch - as soon as a movement commences be mindful that the shoulders, elbows and hands are coordinated and work in concert. At a certain stage of practice movement of the spine becomes under the control of the waist-crotch. In time this facilities the training of waist/crotch power.
The knees and feet - the foot must be relaxed when lifting the knee, at the same time also controlling the span and extent of the shoulders, elbows and the waist/crotch. In time this facilitates the ability to express power from the heels. Mindfully examine, if you make a bow step by stepping forward- are your shoulders loose, your elbows down? Does the waist/crotch relax and move forward? Does the knee and the foot become weight bearing gradually? Is the knee in place? Does the position of the knee stop further movements above the waist/crotch.
Unlike other systems Taijiquan requires the lower body to control and influence the upper body, in order to develop integrated strength and the body as a unified entity.