...WITHOUT USING your brakes...
Listen to the good Captain!
When I trained truck drivers, I used all sorts of silly explanations, analogies, comparisons, parables etc to get points across. One of them was this:
I used to say that going down hill is like hanging your coat on a coat-hanger. Once you have hung it there, it will stay there for any length of time with no further attention from you.
This, of course, requires you to start with a coat-hanger that will support the weight of the coat. A big ankle length fur will never stay on a flimsy wire shirt-hanger -- you'll find the coat in a wad on the floor. (...your vehicle in a wad at the bottom of the hill.)
So that's what the "trucks use low gear" signs are about. It means "trucks use big honking strong coat-hanger". The lower the gear, the stronger the "hanger".
When you begin your "career" pulling a trailer, you may have to mentally force yourself to slow down, and downshift, before the downhill starts. It is something you have never done before, and it does not instinctively seem necessary. But that's what you need to do -- hang the vehicle on a strong enough "hanger" that you do not need to use the brake pedal much to maintain a steady speed all the way down.
Anybody have a photo of that steep grade on 447 by Pyramid Lake? In my four years, I think I have seen a "wad" of a trailer in the ditch at the bottom of that hill every year, when I head down on Tuesday after the event. That's why some of us are harping on this.