Back-up ring is normally installed in a groove behind an O-ring or a hydraulic seal. Its main purpose is to prevent extrusion of the seal when high pressure is applied to the system. The groove to install the ring must be specially designed to fit both the seal and the back-up.

Back-up ring is available in a variety of materials and hardness. For pneumatic applications, a low hardness and flexible material may be used. For hydraulic applications or in a system where a high demand of pressure is requires, flexible back up may leave the seal material vulnerable to being pulled out of its seat and then pinched or torn in the narrow space between piston and cylinder wall. In this case, a stiffer material can be selected.

Besides using the back-up as an extrusion preventer, the back-up can also be used as an aid during installation. For example, it can hold an elastomer seal in place while a machine is being assembled.