In Hunyuan Taiji, Santi is both a static training method and a posture found repeatedly in the form. But the emphasis of this video is static training, which can be easily overlooked in favor of the instant gratification and challenge of dynamic form. However, there is much to be gained from static practice and Santi has challenges and delivers benefits that are in many ways very different from Wuji stance.
A common theme of Xing Yi, Xinyi, Yiquan, and Hunyuan training is intention. Working deeply with Santi is a good way to train intention. In this video, Yang gives a demonstration of subtle movement. This is an important and often overlooked aspect of static training. Once one is comfortable with standing for an extended time, he/she is advised to begin the work of intention. Yang notes that the movements should be smaller than what he is demonstrating. In Hunyuan we go much smaller, to the point of the movements not being visible to the outside observer, or further to only the intention itself--which is deeper and much more work than one would think before learning the practice. But, as we all know, the best way to do it is to do it. Just do it.