The invention relates to an apparatus as well as to a method for expanding preforms into containers within a blow mold by means of a pressurized pneumatic medium, wherein a compressor piston unit for compressing the pneumatic medium is in flow communication with the blow mold, in order to introduce a volume flow of the pneumatic medium into an inner region of the blow mold, and wherein an actuator unit that can be activated by a hydraulic medium interacts with at least one element of the compressor piston unit, in order to drive the compressor piston unit during an expansion process of the container for compressing the pneumatic medium, and in order to be driven itself, during a relaxation process that temporally follows the expansion process, due to the returning volume flow of the pneumatic medium into the compressor piston unit.