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Does anyone know what this hook is for on a Di2 rear derailleur?

It easily rotates 90° by hand.

After rotating it:

4
4 comments

  • Bingo! I discovered that it is an optional wire holder.

  • It’s a clutch mechanism to stabilize the chain, particularly useful on unpaved terrain. It makes the derailleur stiffer and lessens chain slack.

    • Is there an orientation that it should be set to?

      • Afaik, you can leave it in the stiff setting without worry. If the shifting is not setup properly, it could lead to less smooth gear changes. You should deactivate it if you need to set the shifters and move the derailleur - for example to remove the back wheel.

        Edit: the system is called “Shadow Plus”, here explained on the Shimano website.