Abstract: Add-on device (1) to be attached to a lifting vehicle, having a movable fork arm unit (2) comprising at least one fork arm (3), having at least two units (4) for driving a lateral movement of the fork arm unit (2), in particular each comprising at least one drive chain (6), in particular running in opposite directions, wherein the drive chain (6) is in particular assigned to a respective fork arm (3), having in each case at least one hydraulic motor (11), by which the unit can be driven, wherein the hydraulic motors (11) of two units (4) can be connected in parallel for a lateral movement of the fork arms (3) in the same direction and/or the hydraulic motors (11) of two units (4) can be connected in series for a lateral movement of the fork arms (3) in opposite directions. Method for actuating said add-on device (1).