maybe I worded it wrong... say you have a revolver with a external hammer you can leave set on an empty chamber so that if you had pulled the hammer back you can lower it without worrying about it firing.... with hammerless there is no external hammer pull back or drop...every pull of the trigger on a revolver rotates the cylinder and fires the gun so in order to keep from firing it needs set so the first rotation ends at empty chamber.... or just keep your finger off the trigger until you are ready to fire