We're inquiring minds.
We deal with the finite details of bicycle functionality on a daily basis, and one day we were wondering... How does a clutched derailleur actually work? We know the principle of their function, a uni-directional bearing rolls to allow the cage to retract with the derailleur's spring tension, and resists the extension as you shift through the gear range, and as the derailleur compensates for suspension-induced chain growth.
The outer race has pressure applied by an adjustable collar on Shimano, which can be engaged and disengaged via external switch, and on SRAM, via conical bearing seat located within the cage pivot. This we knew, but as to how a bearing could only rotate in one direction, and intentionally bind in the other, was new technology to us. Every bearing we deal with spins forward and reverse--suspension, hubs, headsets, you name it... it spins two directions.
With a demo fleet pushing 80 bikes, we see a lot of wrecked rear derailleurs, and have a bin dedicated to smashed derailleurs set aside for spare parts when the opportunity so presents itself. Enter inquiring minds...
Thank you to Renold Technologies for the informative video below, as well as the images shown here. Their video will elaborate much better than I could hope to explain, plus moving visualizations.