On our two GM diesels, the tow haul mode will change the shirt points to allow for more torque, I believe, as you are gaining speed by holding the transmission in gear before it shifts up. On the first GM diesel, it also had an overdrive lockout on the wand which worked well, but I didn't really need it with the diesel. Haven't noticed whether the overdrive lockout is on this new GM diesel. I don't believe that tow/haul suppressed the overdrive all by itself, it only really did that when the overdrive lockout was activated.
2012 GMC 2500HD D/A Crewcab
2012 Jayco Eagle SuperLite HT 26.5 RLS
2009 Crestliner Superhawk 1900
2013 Polaris Sportsman 550 EXP
2008 Polaris Sportsman 700 X2