Exhaust Manifold Query

[ceatock at sympatico.ca (Chad Eatock)]

Hey there...what is the small pipe that comes off the back of the exhaust manifold on the 16v and capped with some type of heat resistant rubber stopper?  WHen my cat pooched the pressure built up so much between the cat and the manifold that it popped this little cap off this pipe and started making a noise like a crazy gazoo!!  I could'nt get the rocco to go over 80km/h...anyways I just stuck a screw in the hole and blocked it off until I changed the cat...I was just wondering what this little pipe is for?
Chadster '87 16v