the clutch spring grips the internal drum for resistance in a one way action. This is dine through twisting the spring in the direction where the coils tightens up and itoosens up the other way providing zero resistance. That how the whole Retrofriction design concept works.
I had been using and working on Simplex retrofriction shifters since the early 80's (Had them on 90% of all my bikes.) and figured it all out way back then.
What first convinced me on how it works is I noticed that even if the lever has a bit of side to side play (Which means the mounting screw is not bearing down tight on tue lever body to provide friction.) when already mounted on the shifter boss, the lever still provides the resistance/friction it needs to function, without the lever getting pulled back by the derailleur springs. Which means, the mounting screws were not meant to provide any resistance friction, like conventional shift levers do. It is all coming from the clutch spring. It's a very simple concept if you figure it out.....