A Di2 rear derailleur has a so-called RD protection function (RD = Road Dual), a system protection which briefly interrupts the connection between the motor and the joint in the event of a fall or heavy impact on the rear derailleur.
If this happens, you must perform a reset. To do this, press and hold the mode button on the information display (ST2S) or the button on the EW-RS910 (ST5) for at least 5 seconds. This restores the connection between the motor and the joint and resets the RD protection function for the rear derailleur.
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