Changing the volume reducer in the shock is an internal adjustment that allows you to change the amount of mid stroke and bottom out resistance. 

• If you have set your sag correctly and are using full travel (bottoming out) too easily, then you could install a larger spacer to increase bottom out resistance. 
• If you have set your sag correctly and are not using full travel, then you could install a smaller spacer to decrease bottom out resistance.

Look up for the installation instructions and the volume spacer chart.