PT6A Torque Limiter vs Torque Controller

[Aero_eng_81]

Hi,
Can anyone advise me on basic functional differences between torque limiter , found on e.g.: PT6A-60 engine, and torque controller that can be found on PT6A-25 engine. I realize the latter is connected to additional RGB static oil pressure line, but how it "controls" instead of "limits" the engine? Another thing that I'm aware of is that PT6-25C which uses torque controller are used on single-PL-controlled aircraft like EMB-312 Tucano or PZL-130 TC-II Orlik - does it have something to do with this "control" approach?

Thanks in advance