All hydraulic systems operate by transmitting force from one point to another by means of an incompressible fluid (usually some type of oil). The initial force is transmitted to the fluid by means of a piston within a cylinder. As the piston presses against the incompressible fluid, the force is carried by the fluid through sealed tubing to move another piston or pistons at some other place in the system. This is typically how aircraft brakes work. As the pilot steps on the brake pedals, which are connected to the master cylinder, the force is transferred to the slave cylinders which in turn move clamps that squeeze against disks connected to the wheels, slowing their spin by the resulting friction. 

Hydraulic systems can also provide a mechanical advantage which multiplies the original force being applied. If a large amount of movement is needed with a small amount of force, a large piston can be used to drive a smaller one. The fluid moved by the large piston will enter the cylinder with the small piston and move it a distance equal to the volume of fluid divided by the area of the small piston. This principle makes it possible for pilots to move the flight controls of larger, heavier aircraft with high aerodynamic loads on the control surfaces. Without such a hydraulic boost, it would be impossible for pilots to move these surfaces by means of the cable and pulley or push rod systems found in smaller aircraft.

Transport category aircraft employ advanced fly-by-wire systems that utilize electrical sensors to detect pilot inputs and then transfer them to electric power packs which activate the hydraulic system on demand. Hydraulic pressure, stored in reservoirs by means of engine-driven or electric pumps, is released as needed to operate the control surfaces, landing gear, brakes, or other components.