Hydraulic brakes represent an application of Pascal's law: we exert a force with our foot on a small piston and the fluid transmits it and, depending on the relation between the cross sections of the pistons, amplifies it. It also changes the orientation and direction of the force applied.

Disc brakes work in the same way, but in them the pressure compresses some brake pads onto some discs which rotate parallel to the plane of the wheels.

Power brakes
To reduce the effort of the driver when he treads on the brake we use power brakes. With these the foot regulates the action of an electric compressor which amplifies the pressure and modulates the level of braking. It is as if the foot moved a command which gradually freed the action of the compressor.