It supplies fresh air, which contains oxygen, to the catalyst. This oxygen helps the chemical reaction that takes place in the catalyst to operate more efficiently.
In my 1988 Mustang, the smog pump system, called Thermactor Air by Ford, can operate in 3 modes:
Upstream
Downstream
Bypassed
In the upstream mode, used at cold startup, EDIT: after the engine has been idling for 30-45 seconds also, the air is injected at the cylinder heads. This apparently causes the exhaust gasses to continue burning in the exhaust manifolds, since they didn't burn completely in the cylinders due to the cold startup and rich mixture.
In the downstream mode, used after the engine has warmed up, the air is injected at the cats. This air causes the cats to work more efficiently, as I mentioned above.
In the bypassed mode, used EDIT: during deceleration, to prevent a backfire, the air is just dumped out of a hole in the valve just after the air pump.
[ April 07, 2005, 04:14 PM: Message edited by: brianl703 ]