The Jin of Taijiquan refers to internal power. It is derived through the use of the mind consciously directing the movements in spiral fashion, alternating between relaxing and tightening, within the overall requirement of muscular relaxation. In due course, a solid and weighty power develops internally. This power (is not the stiff and brute force) but can be speedily maneuvered and concentrated on any particular point at will. This is what the martial artists term "Neiljin" (internal power or energy).

After one has mastered the external forms, one should timely move to the next stage of training for Jin. The procedure is to build up the Rou (resilient softness) from a process of relaxing both mentally and physically during the practice of Taijiquan and gradually develop the Gang (resilient hardness) from the accumulation of Rou; and from Gang return to the training of Rou. Eventually both Gang and Rou are acquired supplementing each other.

As the Yang school of Taijiquan emphasises on extended forms, light and natural, upright and rounded, simple and stately, Gang is not expressed outwardly. Instead, its Gang is impregnated in Rou. As such, even in the process of accumulating Rou to develop Gang, the muscles stay relaxed most of the time rather than contracting them.

If one fails to move on to training for internal Jin after having attained some foundation in the external form-movements, the result would be a hollow frame of forms without internal substance.

The four basic forms of Jin are Peng, Lu, Ji, An. The Peng Jin is basic to all the other forms of Jin. Peng Jin is an elastic force developed from a process of relaxing. It is not a stiff force, but flexible and spherical. It can not only neutralise oncoming force but also bounce off the opponent.

In Taiji "push-hand", four basic types of Jin are employed, namely, listening Jin, neutralising Jin, controlling Jin and delivering Jin. And of these, the listening Jin is of the utmost importance. "Listening" does not mean to listen with the ears, but rather it refers to the skin-sensing of the opponent's moves.

The classic "Discourse on Taijiquan" says: "From mastering the form-usage one gradually understands 'Dong-jin', and from 'Dong-jin, one is on the way to divine level of dexterity". It further says: "Yin does not detach itself from Yang, nor Yang from Yin and when Yin and Yang complement each other (in one's techniques) 'Dang-jin' is achieved." 'Dong-jin' is the ability to apply the most apt defense-attack techniques instinctively (with minimum effort but the best result).

The final stage of attainment is devine dexterity which transcends forms and emphasises on the cultivation of one's spirit.

This subject can be approached from three angles.

1. Using Yi to direct the form movements.
   One important principle in the practice of Taijiquan is mental calmness and using Yi. The concentrated mind keeps directing the form-movements without a break. Use Yi and not physical exertion" is the exhortation emphasised by most martial artists.
   
2. Using Yi to focus Jin.
   Shift the Yi to the Dantian, keeping the waist relaxed but upright, firm with Chi filled. One will feel the stance is firmly rooted. At the same time, use the Yi to relax the muscles and joints of the back and naturally "HanXiong Ba-bei" is achieved. Use the Yi to fill the hands with Jin and the Jin-focus will keep changing in accordance with the application techniques of the form-movements.
   
3. Conscious of defence-attack techniques.
   During the practice of Taiji routines, one must be mindful of the defenc-attack functions of the form-movements and apply them as if one is engaged in combat situation with an imagined foe.

The inter-play of Yin and Yang brings about changes in the universe as well as in Taijiquan. Changes in the outer forms are preceded by internal changes (i.e. Yi, Chi and Jin in that order). Taijiquan is most adaptable to any changing situation as Yi and Chi are most manoeuvreable, which makes the external movements flowing freely to meet any changes. The sphrerical and spiral-shaped Taiji form-movements easily lend themselves to speedy changes or manoeuvreability.

In conclusion, one must understand the Taiji principles, learn the forms well, understand the Jin, lead with the Yi and apply the techniques dexterously and these are the five ways to excellent in Taijiquan.