Tightening of the nut clamps the tube flare between the body nose (seat) producing a leak tight connection. This clamping on the 37 Degree taper provides a measure of elasticity to the joint helping it to resist loosening under vibration. The clamping force results in a small radical load that tends to deform the fitting nose radially. The resistance of the nose to elastic deformation provides a constant preload (similar to a lock washer) keeping it tight. The clamping force provided by the nut resists the opposing force of the fluid under pressure. The joint remains leak tight as long as the clamping force is higher than the opposing pressure load. Properly assembled Flare Fittings with appropriate tube will seal consistently under pressure until tube bursts.