Owner's Manual
Page 331
... driving on a downhill grade and the driver desires to select a desired range of gears. Using this information, it detects when the truck is enabled (if equipped). See Tow/Haul Mode Light on page 9‑105 for more information. The Tow/ Haul Mode adjusts the... the shift control lever. This feature is active when the exhaust brake is on downhill grades by automatically implementing a shift schedule that utilizes the engine and transmission to reduce shift cycling, providing increased performance, vehicle control, and transmission cooling when towing or hauling heavy loads....
... driving on a downhill grade and the driver desires to select a desired range of gears. Using this information, it detects when the truck is enabled (if equipped). See Tow/Haul Mode Light on page 9‑105 for more information. The Tow/ Haul Mode adjusts the... the shift control lever. This feature is active when the exhaust brake is on downhill grades by automatically implementing a shift schedule that utilizes the engine and transmission to reduce shift cycling, providing increased performance, vehicle control, and transmission cooling when towing or hauling heavy loads....