Did you notice , that the video says"Ultrashift Plus".


The early Ultrashift, that I was around, only used a special clutch that operates like a go-cart clutch.
On that style, at roughly 900 rpms, then the clutch starts to engage. So, if you were to look at used trucks, it may not have the style, that this video showed; SO beware, it might be different.



My opinion, of the early style (Note: I only bob-tailed it a little), was that a guy could possible use the trailer hand brake a little, while he revved up the engine (from idle 600-700 rpms) to that 900 rpm range, to engage the clutch if the truck was at a uphill position, so you didn't roll backwards. That would eliminate trying to use two feet (one on brake and other on throttle) when in that type uphill grade, or slight creep movement. 
Again, I never pulled a trailer, so I never got to try my idea/procedure. I did have to use the "two foot" method, when bobtailing, at an uphill stop. 

After watching that "Ultrashift Plus" video, I believe that earlier version/earlier clutch was less complicated.