Overrun valve trigger (Air Bypass).
It's there to get around the problem that when the throttle is suddenly closed (foot off the accelerator/changing gear etc...) the airflow sensor plate tends to hang in the air whilst the air in the inlet system continues to pass by to fill the depression or vacuum in the inlet trunking between the airflow meter's plate and the now closed throttle butterfly. This holds the Fuel Metering Plunger in the raised position, resulting in over fueling. To get around this, this valve is plumbed in between the bottom side of the airflow metering plate and the trunking at the top side of the metering plate. It opens whenever the throttle is closed to allow direct access for air trapped below the airflow metering plate to get past and fill the void in the inlet trunking.
It is a simple solenoid valve that is either open or closed. It is normally in the closed position, sealing off the bypass, but is activated by the small "switch" on the throttle body that makes contact whenever the throttle is fully closed.
It effectively shuts off the fuel supply by causing the airflow sensor plates to drop back to its idle position.