The secondary air system is designed to add oxygen to the exhaust gases to aid in the final "burn off" of the emissions. Fresh air is pumped into the exhaust by either an electrical/mechanical pump or a special one-way reed valve that utilizes the pulsing of the exhaust from the piston firings to suck the fresh air into the exhaust system. The secondary air is then injected into the exhaust manifolds, catalytic converters, or both.
Secondary air is used most commonly during the warmup cycle to help burn the rich air-to-fuel mixture needed to run a cold engine. Usually after two to three minutes, the catalytic converter is nearly at operational temperature, so the secondary air system shuts down. On some larger vehicles (e.g. trucks and SUVs), the secondary air system pumps air into exhaust manifolds during warmup and then directs the air into the catalytic converters during normal operation to help "burn off" the high fuel loads required to operate these heavy vehicles.