Yep, the Fw 189's props were Argus built, as was its engines. The same was fitted to the Siebel Si 204. The Argus props were controllable pitch, but not constant speed, meaning the pilot controlled the pitch change, or it was done by aerodynamic forces and not a C/S governor.
What follows is purely guesswork, since I've never seen the mechanism of one of these, but I'm assuming the turning vanes would mechanically counter-balance aerodynamic loading on the prop blades. When you increase the power levers, the forces want to decrease blade angle, but the turning vanes would counter this to enable the blade angle to increase or not change, depending on the setting the pilot selects. The vanes would act in the opposite as when the power is brought back, the decrease in rpm wants to increase the angle, but the rotating vanes would counter it. I think this is right; other controllable pitch props work in a similar way using counter-weights to counteract aerodynamic loading.
