Hydraulic dump trucks use hydraulic hoist systems to lift the bed of the dump truck when it needs to be emptied. Princeton University defines "hydraulic" to mean something that is moved or operated or effected by liquid, such as water or oil.
Dump truck hydraulic systems are usually located between the cab of the truck and the body of the dump bed. The hydraulic system used on dump truck beds is made of four main parts: a control valve, the hydraulic pump, hydraulic fluid, pistons and cylinders.
According to NASA, hydraulic pumps work on Pascal's law. Pascal's law states that "when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container." In short, force is applied to one side of incompressible fluid, to move the mechanics on the other side of the fluid.
Standard dump trucks are built on a truck frame mounted with an open box bed that is raised via the hydraulic pump. Standard dump trucks have a short wheelbase which makes them maneuverable. Semi end dump trucks are similar to tractor-trailers that use a hydraulic hoist system. These trucks have large capacity and can dump loads quickly.