Engine braking occurs when the constraining forces within an engine are used to slow a vehicle down, as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes. The term “engine braking” refers to the braking effect that occurs in gasoline engines when the accelerator pedal is released.
Traffic regulations in a large number of countries require trucks to always drive with an engaged gear, which in turn provides a certain amount of engine braking (viscous losses to the engine oil and air pumped through the engine and friction losses to the cylinder walls and bearings) when no accelerator pedal is applied.
Braking causes friction, which in turn causes heat. Too much heat, like what is produced when a big rig tries to maintain speed and control on steep downhill grades, can cause brakes to overheat and fail. When brakes fail, the best course of action is to have a semi-truck roadside assistance you can count on.