A turbo actuator, or wastegate, uses a pressure actuator, which is controlled by boost pressure as to whether it's open or closed, to control boost pressure and power output. When boost pressure builds force, it is applied to the pressure actuator, and when the pressure exceeds the spring value, the actuator opens, allowing exhaust gases to escape, which maintains boost pressure at a set level.
Diversion of the exhaust gases is what controls the speed of the turbine that regulates the rotating speed of the compressor. A wastegate's primary function is to control boost pressure to protect the engine and turbocharger from damage.
There are two types of wastegates: internal and external. Most cars that come stock with a turbocharged engine use an internal wastegate. Internal wastegates are set to handle stock power, and are usually swapped out for external wastegates to accommodate a bigger turbocharger or better control of boost and power output.
An external wastegate requires a specially made exhaust with a dedicated runner going to the wastegate or may be part of the exhaust housing. External wastegates are more precise in high-power situations when high boost is achieved. External wastegates mostly use a poppet-type valve found in cylinder heads and are operated pneumatically. In rare cases, a butterfly valve is also used in external wastegates.