The ARM SAI Filter is a high-flow breather filter for your VW or Audi secondary air intake (SAI).  Most VW's and Audi's are engineered with the secondary air intake (SAI) hose attached to the OEM airbox, which allows the SAI to pull filtered air from the OEM panel filter.  When you upgrade your intake, this SAI hose is left open-ended exposing it to debris.  The ARM SAI filter easily snaps onto the end of the OEM SAI hose using the CNC machined adapter, providing it with filtered air once again.