"Use Jing but not use Force" (用勁不用力) is an important concept in Tai-chi. When we move, walk, pick up something, put down something, we need to exert force. After about the age of 2-3, everybody knows how to do all the above acts without thinking. This is what is meant by using force in Tai-chi lingo. In other words, if a person does the Tai-chi form as if he is moving, walking pick up something, put down something etc. in daily life, he does it incorrectly. In Tai-chi lingo: he is not using Jing and therefore is not doing Tai-chi.
What is Jing (勁) then? Well, one needs to move around doing Tai-chi form, the answer must therefore involve answering the question, how to move-around without being the same as doing our daily chores (walking etc)?
The simple answer is: In Tai-chi movement, the moving part (hand, arm, leg, etc) has to be moved using chi as the primary power generator. It is true that our muscles must take part and exert force in Tai-chi movements (otherwise one can't move at all!), but the leading power should be chi and chi, by definition, can only be directed by one's mind. Schematically, it is like this: mind - chi- muscles, resulting in bodily movement.
Two questions here. First: How to generate chi. Second: How to use chi to direct muscle (the third question is a non-question: How to contract one's muscle to give movement: we learned that after age 2-3).
The best way to generate chi is through meditation. And for Tai-chi practice in particular, the best way is through standing meditation (zhan zhuang) rather than seated meditation, because the former can also chi-condition one's leg muscles which is essential to good Tai-chi practice.
To better direct one's muscles using chi, one has firstly to open one's joints (in particular the four ball and socket joints - shoulder joints and pelvic joints), and secondly to direct the chi through the B&S joints in spiral movement (纏絲勁： Jing like winding silk around). In terms of sense experience, a practitioner will (when he is doing right in terms of using chi to direct movement) feel local internal resistance when the chi passes through the joints. In other words, if one doesn't have such feeling of resistance, one is not yet there.
Needless to say, the concept is easier to understand than to be put into actual practice (or: one can't learn how to do Tai-chi through readings alone!).