How does the oil mist filter return kit work?

The Oil Mist Filter is an accessory that can be used with the Rotary Vane Vacuum Pumps. The user guide for the vacuum pumps can be seen on the vacuum pump item detail pages. One can be seen here: https://www.universalmedicalinc.com/brandtech-vacuubrand-rz9-rotary-vane-vacuum-pump-120v-60hz.html This may better explain how the pump works.