One part of the body can only perform one function
One part of the body can only perform one function. This is an important rule in taijiquan. If followed, the separation of yin and yang will be made possible. Otherwise, all movements are actually one or the same regardless what the practitioner feels, thinks or believes.
Only when the body parts are individually assigned in performing a specific task, can multi-tasking be possible. The multi-tasking is what we call yin and yang.