The invention relates to a method for actuating a reversible belt pretensioner for tensioning a belt strap of a safety belt, which reels off from a belt reel, in a motor vehicle. According to the present invention, said belt pretensioner comprises a drive, which acts on said belt reel and the safety belt comprises a blocking device which acts on said belt reel in the unreeling direction, blocking the drawing out of the belt. The blocking of said blocking device can only be released when the belt reel is turned to a specified rotation angle in the reeling direction. After a triggering of said belt pretensioner, conditional on a dangerous situation and when a value, indicative of the force applied upon the belt, indicates a diminution of said force, an opening mode is initiated. In said mode, the belt pretensioner drive is actuated for opening said blocking device, such that, at the beginning of said opening mode, the belt reel is turned to a specified rotation angle in the reeling direction.