Method for actuating a reversible belt pretensioner for pretensioning a belt webbing, unwound from a belt reel, of a seat belt in a motor vehicle, the belt pretensioner having a drive which acts on the belt reel, and the seat belt having, as a belt extraction lock, a locking device which acts on the belt reel in the unwinding direction and whose locking can be released only if the belt reel is rotated about a certain angle of rotation in the winding-on direction. After a triggering of the belt pretensioner caused by a hazardous situation and if a variable indicative of the belt force indicates a decrease in the belt force, an opening mode is initiated in which the belt pretensioner drive is actuated in order to open the blocking device in such a manner that, at the beginning of the opening mode, the belt reel rotates about the specific angle of rotation in the winding-on direction.