Back Ferrule is used to reduce extrusion. Back ferrule locks on the outside diameter of the tube to complete the sealing action and secure the tube within the fitting. Back ferrule is designed to help compensate for exposure to system variables such as vibration, pressure pulsation and thermal expansion/contraction. Back ferrule is designed to rotate with the nut instead of the lower ferrule, which gets swaged on. This helps reduce torque & twisting action on the tube and/or lower ferrule that actually does the sealing.