That’s not tow/haul, that’s manual mode. Tow/haul is the button recessed into the end of the shifter lever. Tap it and a trailer icon lights up on the dash. That’s tow/haul mode. You are describing manual mode, which is different.
You are correct. Putting it into Tow/haul while in D with the "button" brings in a shifting stabilization mode such as Horse 1 explained. However the manual says if the transmission is shifting too much, drop it down into a lower gear. I don't know if using +/- switch on the shift lever does that while in D, the manual doesn't say, but I do know if you switch to manual, you can. You still have to use the Tow/Haul button. Seems to me, if it is shifting too much,you are going to overheat the transmission as I posted earlier, if you don't use the manual mode. Personally after driving 5/6 speed manuals and diesels going up hills an coming down with heavy loads, I would prefer selecting what gear I am in even though this Duramax has quite a bit more pulling power than my old 98 Dodge. It does work exactly how I described. I double checked the manual to be sure. I will leave it at that and leave others to decide what they will do.
We have an '18 Toyota Highlander with the v6. Near as I can tell, it adds power to the engine but has nothing to do with the tranny.
I had an Acura Legend that was similar. It had a Sport Mode Don't know it it was transmission or engine ,but you selcted it with a button near the shift lever
