Originally Posted by
joejack951
They are based around ratchet mechanisms. When they fail, they miss clicks completely or jam up. The only way to get a partial cable pull would be if only the trim portion of the throw was engaging. But that wouldn't leave the cage multiple millimeters short of where it should go. The only cause for that is, well, I think we've covered that.
Actually, it's a ratchet mechanism that is engaged via a clutch. Part of the clutch is sheared off - so it doesn't engage at the point of the most outward derailleur shift. It does engage over the rest of it's range of travel.