Hoff is Danish.
At the intro of chap three, Hoff explains how he came to use the term "strike pump."
'Systematically different from the bellows gun is a type which is generally called a spring gun, where the momentary air pressure is created by a spring-propelled piston rushing forward in a pump-cylinder. The term spring gun is rather unsatisfactory as it ought to indicate a gun where the projectile is expelled by the direct action of a spring. In the following, the gun with spring-propelled piston will, therefore, be called strike-pump gun.'