As this cross section shows, when the engine is at idle and the throttle valve is closed, intake manifold pressure will be very low. The barometric disc (1) will expand, pushing the iron core (2) out of the gap between the induction coils (4), lowering the inductance, which lowers the voltage to the control box. The spring (3) puts a return load on the core. When the barometric disc contracts under high pressure, the core moves into the gap, voltage goes up -- a signal to inject more fuel.