All bolts behave like a heavy spring. As you turn down the nut against the flange, the bolt stretches and the flange and gasket compress. All bolt-tightening methods result in stretching the bolt. The torque-tightening method uses the thread helix of turning the nut against the reactive forces of the flange to stretch the bolt. The hydraulic bolt-tensioning method utilizes an annular piston threaded on the end of the bolt to provide an axial stretch.