actually it does a lot more than that usually.. its the trucks computer's program that gets modified, and/or "re-flashed". It usually consists of altering the fuel/timing curves in relation to rpm/load, trans shift points based on TPS signal and speed, compensates for changes in tire size, rear end gear changes, reducing and/or removing transmission torque management, pushing the parameters for the MAF, etc... usually done on a custom basis, depending on what mods you have
later