Here is some documentation for the 2014 GM half tons, but it is very similar if not the same algorirthims for 2009-2013 GM transmissions.
With Tow/Haul mode engaged, Sierra’s Trailer Grade Braking is activated when the driver lightly applies the brakes. This feature initiates downshifts to assist with braking on long downgrades. In addition to slowing the vehicle more efficiently, Auto Grade Braking can reduce brake rotor temperatures by 100 degrees Celsius during a braking event.
For drivers who prefer full control over their truck’s shifting, improved TapShift controls on the shift lever are backed by a patent-pending shift algorithm that improves shift response time by up to 60 percent compared to previous systems. When the TapShift controls are in use, Sierra’s Trailer Grade Braking is deactivated because the driver is manually controlling gearshifts.
Source.
I am aware of the grade braking function and that it is disabled in M mode. I prefer to control the gears myself in that situation.
