As a combination of soft movements and internal work, there are two aspects to taijiquan: the physical and the mental. But many practitioners, especially the beginners, have confined themselves to the former to the neglect of the latter.
By the physical aspect we mean the basic movements involved in the traditional routines, done as required for the purpose of relaxing the whole body and building a strong physique. By the mental aspect we mean a cultivation of qi or vital energy through concentration and, in a broader sense, also include cultural refinements. As taijiquan movements are slow and even-paced, there is enough time for the practitioner to exercise his mental faculties at the same time with physical activities.
To begin with, he should do the movements with a big amplitude in order to exercise the body and relax the muscles. As time goes on, his muscles will become more elastic. Then he can narrow the amplitude to save more energy and concentrate on suppleness, agility and sensitivity to the force exerted by the opponent. He should shift his attention from the outward "frame" to the innermost essence of the movements, to the flow of qi within the body, to the mental cultivation of the exercise in order to attain a tranquil frame of mind.
The same principle applies to the practice of push-hands exercises which form an important part of taijiquan. These exercises are aimed to "dissolve" the opponent's force and should be done with a big amplitude in the beginning, involving more movements of the hands than of the waist and legs. When he has gained considerable proficiency, his hand movements will become smaller in amplitude and be well coordinated with those of the waist. Then he has reached a stage where he is able to "overcome a big force with a small force" and "subdue hardness with softness."
Generally speaking, most of wushu exercises are meant to conquer the opponent with action. But a good master of taijiquan tries to subdue action with "inaction," that is, by dissolving an oncoming force with movements of a small amplitude. As a saying goes among the wushu circles, taijiquan pays more attention to the will than to the form. In other words, mental cultivation is more important than physical movements. One who knows only how to do the external movements, but not how to do the internal work will never get at the intrinsic value of taijiquan.
As most taijiquan practitioners cannot afford much time even for the physical aspect of the exercise, they are required to learn the mental aspect at any moment in their daily life -- not only when practising the movements, but also by concentrating on whatever they do and keeping themselves completely relaxed while at rest.