A piston air rifle works like a spring powered rifle, except that the power source is a coiled metal cylinder, or "piston," rather than either compressed gas (Nitrogen) or a mechanical spring. In the modern version of this mechanism, the power for cocking and recocking is provided by an integrated form of pneumatic booster based on a small, high-pressure air reservoir. When the shooter pulls the trigger, the sear releases a catch that allows the piston to drive forward. This compresses the air in front of it and forces the projectile out of the barrel. As the piston moves back, it draws fresh air into the gun from behind the barrel. This recocks the gun, ready for the next shot.