safety belt automatic wind up devices having an energy converter having a charge which when activated in the case of danger the explosive pressure of the fired charge acts on a retightening element which tightens the safety belt around the body of the motor vehicle passenger the retightener of the invention has an intermediate liquid medium transmitting the explosive pressure of the charge arranged between the charge and the retightening element application_number US 12749480 A description an intermediate liquid medium which transmits the explosive pressure of the charge is arranged between the pyrotechnical charge and the retightening element the liquid medium may be any suitable liquid which is non corrosive in the container and which desirably has a low freezing point liquids commonly used as hydraulic fluids are generally satisfactory the liquid may be water with an additive to lower the freezing point or may be an organic liquid such as a glycol by using an intermediate liquid medium according to the invention the sealing requirements for a linear tightener are reduced while the efficiency is increased in a rotary tightener only the intermediate liquid medium makes a transmission of the explosive pressure of the charge to the turbine wheel possible with sufficiently high efficiency in retighteners with a turbine wheel as the retightening element which is preferably mounted directly on the shaft of an automatic wind up device taking up the safety belt a container is provided which receives the liquid intermediate medium and which.