tmajor

2012 STX

I believe, the salesman told me, it closed the engine valves or something, as I said to him, "It sounds like the Jake Brake system".

The other day, as a test, going down a hill, I engaged it, even though I wasn't hauling or pulling a trailer. Once I tapped the brakes, the engine did go into a "hold back" mode (different sound), until I pushed on the accelerator.

smokeywren

There is a decent explanation of the tow/haul mode in your Owner's Guide - in the Driving section under Automatic Transmission Operation. In my 2012 manual, it's on page 300. In a nutshell, it delays upshifts to reduce frequency of tranny shifting, provides more engine braking when your foot is off the go pedal, and downshifts when the trailer is pushing you down the mountain.

I don't notice it on mine when towing on the flats. It's only gets noticeably active when you are coming down the mountain. Or when in hills, it won't upshift to 6th gear until after you are going down the hill for a spell. It spends a lot more time in 4th and 5th gear than when tow/haul is not engaged.

Triggerfx4

Basically what the manual says is right how it's been explained to me is it is for trailer towing. When your not in tow haul mode your transmission shifts faster ( at a lower RPM) and tends to keep the truck in a higher gear to save on fuel. When in tow haul it tends to stay in a lower gear and not shift as quick( at a higher rpm) to have more towing power and pull easier and basically keeps you transmission from shifting up and down as much which would wear out the transmission a lot faster.