This patent describes Campagnolo's Portacatena (which roughly translates as 'chain carrier'). It works like this:
So far, so clever, but there was only a small swarm of flies in the ointment. You had to set up the rear derailleur with its top sprocket limiting screw slackened off to allow it to move far enough to shift the chain onto the Portacatena. Then as the gear cable inevitably stretched in normal use there was little to stop you accidentally changing gear into top gear and then onto the, nearly frictionless, Portacatena. This was not exactly perfect when you were just sprinting for that 30 sign. Of course the Campagnolo Nuovo Record and Campagnolo Super Record derailleurs of the time were notorious for being especially imprecise at changing into the top sprocket - so fitting a Portacatena just added the icing to an already rich and flavoursome cake of problems.