In Yiquan, when you train in zhan zhuang postures you use mental visualizations that aim at developing gradually some special sensing ability and internal power by fusing your body and mind. Visualizing means here actualizing, not merely using imagination.
When you have some good basis in zhan zhuang you go to shi li, which is an attempt to keep the internal state of feeling force, power movement in non-movement found in zhan zhuang into some slightly moving postures which means finding the non-movement into the movement and keeping the feeling of force.. You achieve this through movements accompanied by visualizations (images, feelings, forces applying to your body) that generate specific sensations. This is testing force/shi li. When good at it, it looks like what you have seen in the vid.
Difficult to explain but hope it helps
yeah that is difficult to explain, but i get it, if only intellectually. thanks