The air injection system uses a simple principal to oxidize or burn HC and CO. If you have HC and CO in the exhaust stream, you have fuel left over from the combustion chamber. Since you have leftover fuel (HC and CO) in the exhaust stream, you still have heat (If you have ever burned yourself on an exhaust manifold, you know that there is still heat there), all you have to do is air or O2 and you have combustion all over again (fuel + air + Heat = Combustion). Air injection is simply a continuation of the burning process to burn up fuels that did not burn in the engine combustion camber.
Common components of air injection system are as follows air pump, diverter assemblies, and the check valve. The air pump directs a light pressure of air (O2) through the pipes into the exhaust manifold and/or the catalyst. The air pump does not have to move a large amount of air because little air is needed to start the burning process of fuel left over. Many air injection systems do not uses an air pump at all; pulse air systems, or aspirated systems that use exhaust stream "pulse" that vacillate depending on whether the engine is in the power or exhaust stroke. The pulse air injection system uses the "draw" to pull the fresh air through the check valve an into the exhaust stream. Diverter assembly one side effect of pumping air into the exhaust stream is when the vehicle is decelerating under engine load. During these conditions, it is highly undesirable to air pumping into the exhaust stream. Pumping air into the exhaust stream while the vehicle is decelerating under engine load usually results in a backfire. These backfires can be strong enough to launch parts of the exhaust system off of the vehicle. The diverter assembly can detect when a vehicle is decelerating under engine load and will prevent air from being delivered into the exhaust stream and dumped into the outside air. In this way, the diverter assembly prevents the engine backfires that could result in exhaust system damage.