Actuator

"A wastegate is simply a device that bleeds off exhaust gas before it reaches the inlet of the turbine housing.
As exhaust fills the manifolds, it is directed toward the turbocharger and enters the turbine housing before expanding across the turbine wheel and exiting through the downpipe. In a closed system, the turbine would see all of the exhaust throughout the engine’s operating range and boost would continue to rise, uncontrollably, until either the throttle was shut or the turbine wheel reached its choke point. For most engines, this would create an excessive amount of boost/airflow and destroy parts, leaving you with a couple of melted pistons at best or a giant hole in the block (much more likely). To control boost and overall engine power, turbocharger systems rely on wastegates, which are mounted before the turbine housing (or inside of it in the case of an internally-gated turbo), and act as a controlled bypass for a percentage of exhaust gas to regulate turbine speed and, thus, overall boost."