Fuel enrichment relay in an '85 TD? IIRC fuel enrichment is mechanical with a vacuum switchover valve in the circuit to cut the boost signal to the fuel [de]enrichment device.

As to how it works, there's a device atop the IP called the ALDA. Absent a boost signal from the intake manifold, the ALDA presses down on a plunger in the IP to restrict fuel to about as much as a naturally aspirated engine can handle. As manifold boost builds, a boost signal is piped to the ALDA which responds by releasing pressure on the IP plunger, allowing more fuel in relation to a denser air charge into the cylinders. The overboost circuit has a 1.0-1.1 bar switch in manifold. At the setpoint, presumably because the turbo has lost its ability to limit to boost to levels safe for the engine, the switch completes a solenoid circuit within the switchover valve to cut the boost signal to the ALDA. The engine is essentially limited to it's naturally aspirated equivalent's fuel supply until manifold pressure drops below the overboost setpoint.

Sixto
87 300D