I believe is was a Chevy pulling 21K/4K TW and a B&W. It's not my video, just read the comments.
Here is another that is not a B&W.....
https://www.youtube.com/watch?v=CwTZqMvcaLM This one has the old school rail mounting system.
I'm no engineer, but I'm sure a certain amount of flexing has to happen or there will be a fracture of trucks frame or the fasteners. B&W's base plate is made with materials that are heavy enough to handle the flexing for many cycles. If it were made with rigid tube it would most certainly fail in short order.
After seeing the base plate flexing in the second video, one can visualize what would happen when you add 12" post and raise the hitching point 14" above the gooseneck ball. The forces that leverage creates has to show up somewhere.
Again, this post wasn't to trash anyone's product. B&W is my preferred product for this application. Many people here told me this flex was impossible with a properly installed hitch, so I made a video to show that it is.