Grade Control
While a vehicle is driving up a hill, gear position is fixed to 3rd gear for avoiding busy up and down shift between 3rd - 4th gears.
When a vehicle is descending a steep hill under the designated vehicle speed (approximately 50 miles/hour), 4th gear downshifts to 3rd gear automatically by depressing the brake pedal.
This gearshift control is released by re-accelerating with depressing the accelerator pedal.
These controls are doing this based on the combination of throttle opening angle, engine speed, vehicle speed and so on.