Here's a cutaway of Progressive's 422 Series shock. The shock works with a combination of a spring and damping piston/oil system and high-pressure gas. While the spring takes up the initial hit of the bumps, to help avoid the pogo effect of the spring, the internal damping system smoothes out the compression/rebound for a plush ride. You can also see the preload and ride-height adjustment points. You may also notice that this cutaway is of a unit for '00-later Softails. The only difference between the '89-99 unit and the '00-later is the swingarm mounting point (arrow).