This is used to help with filling 41mm forks with oil when mono tube upgrades are installed. Normally this process takes a lot of your time to do as there is little to no room between the mono tube and the inner diameter of the fork tube so you have to pour the oil in very slowly. This uses a stock fork seal to seal against the outside of the tube and then you just pour the amount of oil you need in the funnel and just let it sit and slowly fill itself while you continue working.