Coming from the East Coast we don't deal with the grades found all over the West. On route to Yellowstone in 2005, from the flat lands we climbed, if I remember properly, the Big Horn Mountain. We were busy negotiating the switchbacks and looking back at the vast plains we had just crossed. When we topped out and I looked at the descent before us, I was scared we would burn out our brakes as we drove down.
When our 2004 F-350 diesel was delivered the East Coast Dealer was very vague about what the Tow Haul system was all about. They said when your towing something it helps with the shifting and like that. I hit the tow haul as we began the go down the mountain and we descended at about 25 mph and barely needed the brakes. There after a Ford Dealer in Cody, WY explained the workings of T/H and our many mountain adventures after, were easy and safe. We now use T/H when ever going down steep grades.
Our rig on that trip, which was to Alaska, weighed in at a little under 7 tons. With out the assistance of downshifting and exhaust gas back pressure we would have been in deep do-do. Diesels do not have the back pressure of gas engines on downshift. T/H directs the exhaust through the turbo to add back pressure to the engine, was how it was explained to me.