A California emission smog pump.
I havent ever seen one before, what is it connected to and how does it work?
You probably have seen these on something, but it didn't register at the time. They are a popular delete item in the performance world. Generic smog pump information here. They came into use in the late 60s.All manufacturers used them at some point.
Until the catalytic converter they were used to pump air into the exhaust port to burn off combustion emissions, much like blowing on the embers of a weak fire to get it burning better. All systems use a check valve to stop exhaust coming backward to the pump and a valve to prevent backfires.
After the catalytic converter was introduced, they usually were used to blow at the exhaust port and to feed air to the catalyst so it could burn off combustion emissions (HC,CO). These systems had a switch over valve to control where the air was pumped (upstream,downstream)
They are still used today, but are usually electric and only blow to the exhaust port during warm up. Some of them are pretty hard to find as they are well hidden and much of their plumbing is cast into the heads.
They don't use much power and don't blow much pressure. The chief performance concerns are the extra heat at the exhaust valve possibly leading to detonation or burnt valves. If the engine is modified the anti backfire valves were usually vacuum controlled with ported vacuum and might not prevent all exhaust backfires. They also could cause a catalyst to overheat on very rich running cars, think lots of fuel and air, burning under your carpet, with just a few layers of sheet steel for protection.
They actually were first used on some performance models, the earliest I know of was the original z28 Cali package. While they don't really hurt performance themselves... the couple horses they consume and the above concerns made them an immediate removal item for the racey types.