That isn't the perfect video, but for very young subjects and only 44 seconds in length it says a lot. I can't arch like that, but I do maintain some - for sure. There is more detail not really covered, like pulling your shoulders back to remove an inch or two from the bench, and remove some load from them (shoulders). Push your traps into the bench with your feet, this helps keep the arch and keeps you tight. Feet not too wide, not too narrow, gotta have stability.
Wrist action should be saved only when you need another half an inch to touch your shirt, again these are kids so you have to forgive them.