A bicycle derailleur comprising a chain guide, an electric motor that displaces the chain guide and an electrically controllable device configured to, selectively, block the rotation of the electric motor and let the electric motor free to rotate. A method for actuating a bicycle derailleur comprising a chain guide and an electric motor that displaces the chain guide, comprises the steps of: receiving a gearshifting request signal; making the electric motor free to rotate; driving the electric motor to displace the chain guide until the chain guide is in an intended position while the electric motor is made free to rotate; and, blocking the rotation of the electric motor.