Explanation

"A vehicle fitted with an automatic transmission usually changes to a higher gear as its speed increases and to a lower gear as it falls. It will also change down to a lower gear going uphill, as the load on the engine increases. However, there are times when you need to stay in a low gear even if the speed is constant and the load is light - for example, when going down a steep hill. The system normally tries to change to a higher gear in these situations; the driver is usually able to override the system by using the gear selector to stay in a low gear."