Its the cylidrical piece on a internally wastegated turbo that sits long-ways on the turbo. It has a hose running from a tap on the compressor housing to the cylinder. It then has a rod coming out of the end of it connected to the flapper on the turbine housing. When boost is present, at a preset boost level, the actuator opens the wastegate flapper allowing exhaust to bypass the turbine impeller. It just controls(or activates) the wastegate flapper.
