Originally Posted by
ThermionicScott
I thought Justin and Auchen covered it pretty well. The spring add resistance against (backward, retro) the pull of the derailleur spring to lighten its action.
As for me, I like my "friction" shifters to click a little. The micro-ratchet allows the shifter to hold its position without requiring as much friction as the older-style friction shifters. Thanks, Shimano and SunTour!
Right on, it didn't seem as though there was a definitive source for the information, but it does make sense the way auchencrow explains it.