To answer your question there is actually a gyro in the car that senses the angle of the car (i.e. through the gyro the car senses G forces when you are turning, or going up or down a hill). The transmission works in conjuction with this gyro to control it's shift points. The reason it down shifts when you are going down hill is because the gyro senses your position pointing downward and then tells the transmission to downshift so that you can use the engine as a break. The opposite happens when going up hill, the gyro tells the transmission you are going uphill and thus it holds gears longer to help you make the climb without shifting to the next gear and thus being out of a usable power range. Also on the other side, the car can sense when you are coming out of a turn and will hold gears longer to accelerate away from turns. Pretty neat technology huh?