Originally Posted by mocohead
I believe that is the vacuum operated wastegate. When vacuum is applied, it opens the flap on the turbo to release boost.
You've got it.
My car has an N26, so this is electro-mechanical rather than vaccuum operated, but on the standard N20, BMW uses a pretty run of the mill twin scroll turbo w/vacuum-operated internal wastegate; only difference is the operation between N20/26.