Backup rings are commonly installed with rubber O-Rings to help improv  their performance. These rings are designed to limit o-ring clearance gaps and be used in high pressure sealing situations greater than 1500 PSI. Using these rings will prevent the o-ring to become damaged or extruded causes failure of some systems. Generally, backup rings come in two different shapes, flat-sided and concave. The reason behind the introduction of the concave backup ring was to relieve the pressure off the O-Ring as it deforms under pressure. However, there are no real performance benefits from either a flat or concave backup ring.