There is a certain subset of our motor neurons, called mirror neurons, that activates in response to the movements of others as if we were making identical movements. When you see another person, these neurons create an internal representation of that person’s posture, movements, sensations, and even their emotional state.
Normally these signals from our own nervous system are very subtle and faint, so that we are unaware of them, but they may be very strong at times, as when seeing another person become injured compels you to feel their injury in your body, and flinch as if to avoid it. They are also responsible for emotional contagion, the effect whereby moods are transferable.
In tai chi we train ourselves to be more sensitive to these signals, among other things. By stilling the body and quieting the mind, we become attuned to the subtle play of neurological impulses resonating within us, including those that link us to others, allowing us to both feel and follow their intent. The practice of mirroring is one way of directly exercising this capacity. Standing in front of a partner, take turns mirroring one another’s slow, gentle, continuous movements. At first there will need to be a leader and follower, but with practice and attunement you can eventually drop the roles and engage in free-form flowing together.