In this episode of Wolverine Airsoft's "What The Tech?!" they talk about FRAC for HPA use... "The FRAC (Fixed Rear Airline Connector) was introduced on the MTW-308 and provides a hard-point airline fitting on your MTW’s sling plate, removing the need to run your airline out your grip and allowing for one-handed connecting/disconnecting of the airline.

Glenn installs FRAC on an M4 on this episode of What the Tech!"