A Di2 rear derailleur has a so-called RD protection function (RD = Road Dual), a system protection that briefly interrupts the connection between the motor and the joint in the event of a crash or a heavy impact on the rear derailleur.
If this happens, you need to perform a reset. To do this, press and hold the Mode button on the information display (ST2S) or the button (see image) on the EW-RS910 (ST5) for at least 5 seconds. This will restore the connection between the motor and the joint and reset the derailleur protection function for the shifting mechanism.
