Fox Float CTD/DPS Shock 9mm Shaft Air Volume Tuning Kit

Changing volume spacers in the shock is an internal adjustment that allows you to change the amount of mid stroke and bottom out resistance. By decreasing the volume in the air canister, the shock will ramp up more quickly at the end of its travel, resisting harsh bottom-outs while still providing small-bump compliance in the top of its travel.
It is critical to follow Fox's instructions and recommendations of which spacers to use.