As emission control strategies grew more sophisticated and effective, the amount of unburned and partially burned fuel in the exhaust stream shrank, and particularly when the catalytic converter was introduced, the function of secondary air injection shifted.
Rather than being a primary emission control device, the secondary air injection system was adapted to support the efficient function of the catalytic converter.
When the catalytic converter is cold, air injected at the upstream point burns with the deliberately rich exhaust so as to bring the catalyst up to operating temperature quickly.
Once the catalyst is warm, air is injected to the downstream location — the catalytic converter itself — to assist with catalysis of unburned hydrocarbons.
This valve senses the sharp increase in the intake manifold vacuum resulting from the sudden closure of the throttle, and diverts the air pump's outlet to atmosphere.