This is the wastegate flapper valve. When it is closed, all the exhaust gases must pass through the turbine. It is opened to bypass exhaust gases away from the turbine wheel to limit boost.

Since the 13C is such a small turbo, the wastegate does see a lot of work at cruise speeds and when the engine is under moderate to heavy loads.

The shiny circle around the flapper is where the flapper contacts the housingwhen it is closed. The dark spot in the middle shows the size of the wastegate passage. There is room to open up the size of the passage and not compromise the closed flow stopping capability of the flapper.

Opening up the wastegate passage to improve the volume of the bypass flow is known as "porting the wastegate". I've done the procedure on Garrett turbos, with excellent results.