A portable linear fastener driving tool (10,401) is provided that drive staples, nails, or other linearly driven fasteners. The tool uses a gas spring principle, in which a cylinder (71) filled with compressed gas is used to quickly force a piston (80) through a driving stroke movement, while a driver (90) also drives a fastener into a workpiece. The piston/driver is then moved back to its starting position by use of a rotary-to-linear lifter (100), and the piston again compresses the gas above the piston, thereby preparing the tool for another driving stroke. The driver has protrusions along its edges that contact the lifter, which lifts the driver during a return stroke. A pivotable latch (120) is controlled to move into either an interfering position or a non-interfering position with respect to the driver protrusions, and acts as a safety device, by preventing the driver from making a full driving stroke at an improper time.