Tai-Chi essential 6 of 10: 用意不用力 (Mind leads movement)
用意不用力 太极拳论云：此全是用意不用力。练太极拳全身松开，不便有 分毫之拙劲，以留滞于筋骨血脉之间以自缚束，然后能轻灵变化，圆转自 如。或疑不用力何以能长力？盖人身之有经络，如地之有沟壑，沟壑不塞 而本行，经络不闭则气通。如浑身僵劲满经络，气血停滞，转动不灵，牵 一发而全身动矣。若不用力而用意，意之所至，气即至焉，如是气血流注， 日日贯输，周流全身，无时停滞。久久练习，则得真正内劲，即太极拳论 中所云:“极柔软，然后极坚刚”也。太极拳功夫纯熟之人，臂膊如绵裹 铁，分量极沉；练外家拳者，用力则显有力，不用力时，则甚轻浮，可见 其力乃外劲浮面之劲也。不用意而用力，最易引动，不足尚也。
My rendition of essential six: “During practice, always use the mind to direct (the whole body’s) chi to the point of movement rather than only engage the specific muscles involved.” This essential act can only be achieved after the whole body has become loosened and all blockages cleared– done through the practice of essential 1 – 5. In addition, the body must be in a relaxed state before the mind can direct (the whole body’s) chi to the point of movements.
Paul’s additional comment: This is the quintessential mind-body exercise form of tai-chi. The transmission route is through body relaxation plus chi being directed by one's mind. As the practitioner progresses, his (restless) mind will follow the chi movement. For a proficient practitioner, movements run smoothly and continuously (one follow the other smoothly). After further practice, his chi will be at one with his mind, in a state of moving meditation – the ultimate Tai-Chi experience. This development process of mind and chi is conceptually the same as Taoist meditation (or Taoist yoga. The difference is that the practice of Tai-Chi will result in more physical change whereas the practice of Taoist meditation will result in more spiritual or psychological (personality) change.