Concerned with body positioning, balance, weight, speed, contracting and relaxing the muscles, calming the disturbed emotions and quieting the thinking mind. Then using these skills to physically interact first with a partner, then later life. Perhaps 4 to 8 years of internally aimed external effort is required to open the door to the true internal, depending both on the initial condition of a persons system and the intensity and intelligence of their practice. Beyond this, many do not go. Progress from External to Internal begins from the very first exercise in true Taiji training. It is a mistake to aim to strengthen only the external first then hope to approach the internal later.
Attachment to Body
Unfortunately, when Taiji is taught and learnt primarily for the purpose of healing (with emphasis on soft, pleasant outer conditions and little effort required from the practitioner) or overemphasising self defence (which while requiring effort thereby strengthening the body and personality structure, also inevitably builds the outer ego), then reliance on and attachment to the body is increased, suppressing the influence of the small inner voice and its natural innate aspiration towards the Beyond, thereby further obstructing the already narrow door to true internal progress.
Motive, Effort, Help
Progress towards the threshold of the Internal depends on purity of motive, intensity of effort and help from the teacher. These are the 3 important factors that will irresistibly lead to deep development. Intensity of effort causes change on a deep level, which unfortunately also stimulates growth of the ego - often on a subconscious level. Only purity of motive will ensure that the inner-growth strengthens the hold of the True Self, rather than the false self (ego) which if left to grow will bring all internal progress to a halt.
To deliberately direct all results of inner training to the benefit of the True Self, then consciously remember this aim when beginning to practice or whenever results seem to appear, is the base method for gradually purifying inner motives. Only to the degree that you have purified and understood your own inner motives will the motives of those around you become clearer. Until that time of understanding arises try to see others in a positive light or the ego will entrench itself by placing you at a higher position than you really are while placing those around you lower - this being the most obvious outward sign of the ego at work.