Double Slips are used in applications that are under pressure (i.e. turbo-manifolds), or where minimal leakage is required (i.e. up stream of an O2 sensor or in a street car where an exhaust leak would be unacceptable.). The Double Slip does three things at your joint seam; it forms an excellent seal, it provides flexibility, and allows for disassembly. These are often great to use in place of flex bellows.