It doesn't happen in that video either.
When they go to what you call light sparring there is just loose striking and then clinch. Somewhere between this range is where supposedly he should be establishing a "bridge" and then using "force flow" to unbalance the guy and bounce him around while striking him.
It never happens.
Even if it did in light training with a likeminded person, it is still never seen in their actual fights. All we see is free striking and grappling, because that's what actually works.
There is a very important difference here.
This force flow thing is said to be used directly in fighting. The end goal is to be able to manipulate the opponent's body and balance while striking them. But it never happens.
Training drills in WSLVT are abstract, not meant to be used directly in fighting. We train pun-sau to develop the punch. Pun-sau doesn't happen in fighting, but punching obviously does. There's no disconnect between what we say we use in fighting, and what is actually used.
If you are blind to it then not much we can show you. WSL Wing Chun is not the same as what we do. I know that style well. Its not the same approach. Once you have first hand experience then maybe you will feel the difference