I have a 1990 ford f150 with the 302, yesterday I was looking for an exhaust leak and come across a rusted end coming out of the pipe just after the header

What does this do?
it just leads back towards the engine bay and connects to something that only seems to blow air.
Just wondering what it even does.

 

That's the muffler for the bypassed secondary air. Click this & read the caption:


(phone app link)