Most practitioners of tai-chi have heard about the concept of Brute force (Li-力) vs. Jing (勁). The conventional wisdom is that tai-chi uses Jing instead of Force (Li). Some can express the use of Jing in doing the form, some cannot. But little is the number of practitioners who know how to express it and explain it. I'll try to explain it in this post.
All force is defined as F=ma (force equals mass times acceleration). There is no difference between brute force and Jing under this definition. To increase F, one must increase either or both of m and a. This is the law of physics, nobody can do anything beyond this constraint.
Firstly, let us tackle the variable m, mass. And let us use the action of PUSH as an example. In tai-chi lingo, brute force or Li (力) is defined as the sole usage of the arm muscles (in particular triceps of the upper arm) in executing the push action. The mass will (only) be created by the arm muscles used. On the other hand, as the argument goes, in the execution of Jing, in addition to one's triceps, other muscles will also be activated, hence mass responsible for the push force will increase. And in the limiting case, all extendable muscles that can be directly or indirectly made to connect to the triceps will be linked together as a single mass. How to do it and to what extend can the muscles can be linked and contracted together is another issue to be investigated. The conclusion is clear: the use of Jing will involve a greater mass, and everything being assumed to be equal, a greater mass will generate a greater force.
The next problem is consequently how to link the muscles together to build up a bigger mass. The best way for a practitioner to feel some of his muscles are being linked together （肌肉如一) is through doing Zhan Zhuang, before doing the moving Form. The details of which can be obtained from my previous Zhan Zhuang posts. This is the area of execution.
Secondly, let us tackle the variable a, acceleration. When you see doing tai-chi, they are almost always in slow motion. How can one train acceleration through slow motion? You, as well as many tai-chi practitioners wonder the same. Seasoned meditators can appreciate that one important objective in chi-meditation is to have chi filling up one's body evenly. In other words, all muscles will be filled or saturated with chi. Again we shall use pushing action as an example. In would-be pushing action, one will contract one's triceps and relax one's biceps. In meditation, all muscles will be activated (i.e. in contracting mode), in our case, both the triceps and the biceps will be activated or in contracting mode. If both forces are equal, there will be no motion, or, in dynamic equilibrium. The most vivid example is drawing the bow. In dynamic equilibrium, the arrow will have no force acted upon. Energy stored in the bow will, however, be transferred to the arrow when the string is released. In tai-chi lingo, "Activation of Jing is like shooting an arrow" (發勁如放箭).