this is called volley jin. Zhang, Shifu demos in this way to allow others to feel it. Often talked about on the web, known by other names “kong jin” or what some call “empty force”
Often the subject of many heated discussions on the web, and even there watching, feeling it first hand. Many had different outlooks reflecting the training back ground of the student. Many of Master Zhangs’ students have come from many years of training in other Chinese martial arts before meeting Master Zhang, and beginning their own Journeys to emptiness.
I will add some other clips here from different teachers, with my own commentary hopefully helping to explain what is viewed. This has been a part of my own research, studying , training with Master Zhang and the taiji family there.