I believe the general concept of the retrofrictions is they give a bit more resistance when you shift to a smaller cog. The spring might have something to do with that. Someone may be along to correct me shortly.

I do know they are far and away the best shifting friction shifters I've ever ridden with. At this point, I have them on four seperate bikes; I upgrade the shifters on my riders as I find the retrofrictions. So the 1972 PX, the 1979/80 PK and PF, and the Trek 560 all ride with them.