A combustion driven fastener hand tool is disclosed having a piston driven by combustion within a chamber, the piston affixed to a driver for forcing a fastener. The piston forms a retention chamber having a lower internal pressure (vacuum) than that of the surrounding combustion chamber when the piston is disposed in the starting position for operation of the device. The lower internal pressure of the retention chamber (generated from a previous combustion cycle) momentarily holds the piston in place such that combustion behind the piston must first increase sufficiently to overcome holding force of the retention chamber before the piston moves. Requiring build up of combustion chamber pressure before piston motion greatly improves efficiency and power of the device. Use of the vacuum retention chamber is readily tunable with proper sizing of the chamber.