When Chen Weiming (a famous author and close disciple to Yang Cheng Fu) wrote the 10 essential points based on Yang Cheng Fu's teachings at the time, “use Yi and not Li” has probably been the most misleading and confusing for many Tai Chi practitioners today.
This Tai Chi classic is roughly translated as “use the mind not strength.” It is understood as Yi representing mental intent and Li representing physical strength. The problem occurs in the interpretation of the word Li.
The word “Li” is only separated here in the western world. In Chinese, “Li and Jin” are interchangeable, where Jin represents internal strength or force. For example, when my Tai Chi teacher used to say "your feet need to have Li" sounded confusing for Tai Chi practitioners who did not understand Chinese, but in Chinese, having Li there means you need to have strength in your feet and legs. When you train Gong Li (strength skills or ability) for your legs, you are using your leg's strength. This doesn't mean you are applying Li as a muscular force, but instead having Li (strength and force) within your structure. Li and Jin are very important in Tai Chi training.
Unfortunately, the classic “use Yi not Li” has mislead many. Because of this classic, too many people over focus on relaxation. This results in practitioners training in Tai Chi with high stances and small movements just because they are overly focused on relaxation and being soft, and therefore afraid of using Li. My Tai Chi teacher used to say all the time that you need to train Gong Li by working through your stances in your form practice. He told me that “In old days Tai Chi students trained Gong Li by using heavy weapons, stones, push hands and stances.” The upper and lower body strength was a must in the old days.
Nowadays when people talk about Li in Tai Chi, they are confused and say, “You absolutely cannot use Li. If you do, you have no clue about Tai Chi. Li is used in Kung Fu (external training) not Tai Chi (internal training)!”
When Chinese martial art masters speak of Chinese martial arts and they speak of Li, they are using Li as the equivalent of Jin. Within the Chinese martial arts training, Li is the same as Jin. Both are the same product of the training, but if you have a non-trained person, they can only use their natural Li, which is untrained and therefore by definition not Jin.
All Chinese martial arts train full body integration as the underlying factor to their techniques and skills. Integrated body movements are more important than emphasizing local movement (such as punching with only the arm movement and not integrating the whole body into the punch).
I remember my Tai Chi teacher saying that "Li" itself just means strength, power or force. It doesn't translate to "muscle strength." He was not sure who started the muscle strength translation, but that is not the meaning of Li. The problem is that, like Song, there is no exact translation for Li into English.
Therefore, to use Yi not Li basically means to focus on your mind/intent and not on your force/strength/power. The translation should be more like focus on the Yi part and you won't go wrong with the Li part. This is because using your Yi is the correct way, making it not as necessary to fully understand Li. Sometimes you don't need to fully understand something to fully achieve it. Just understand the main principle, which is to use Yi, and follow that training method. This might sound strange to westerners because when you hear the translation "use Yi not Li," you can't help but automatically think that you should not use Li at all. The better translation would be: "focus on Yi not Li."
To further clarify, it is not that Li is not required. In fact, some of the old masters used to say, “better physical strength will have better jin.” Basically this means there is correlation between physical strength and Jin. That is why there is emphasis on strengthening the lower and upper body in old days. However, the glue is always Yi. In order for the Jin to come out, you need to use Yi.
The phrase "Use Yi not Li" is an exaggerated statement that is not meant to be taken literally, similar to the saying “this bag weighs a ton.” It doesn't actually mean the bag weighs a ton, but that it is just very heavy. In all Chinese martial arts you need to train your legs and body to have true strength. Even in Tai Chi which focuses on not using force, you still need to train Gong Li for power. Not using force doesn't mean being weak.