In taiji, we keep talking about not using force. But "not using force" should not be taken literally. After all, taiji is a martial art, and there is no martial art that does not use force. The term "not using force" is actually a technical term that is used to set taiji apart from other forms of martial arts.
Instead of a more or less continuous application of force, taiji is about the selective and minimum use of force. The least amount of force is used to achieve the desired effect. This is the minimum part. In order to allow this small force to have the maximum effect, it needs to be applied at the right time. This is the selective part.
It is this selective and minimum use of force that allows an old taiji practitioner to still be able to neutralise attacks. An old person is definitely not as strong as a young one, but by using whatever little force that old person has at the right time, he or she is able to change the trajectory of any incoming attack to render it harmless.
And that is why we train. Only through training can we learn to recognise what this "right moment" is, and what the "minimum force" required is. The use of the term "not using force" during training is a reminder to ourselves that we are here to find that right moment and minimum force. It is "not using force" that sets us apart from other forms of martial arts. "Not using force" is not literal; it is the name given to a concept, the concept of selective and minimum use of force. It is only through understanding the concept behind the label that we can practise correctly toward the desired ideal state.