...mainly by taking advantage of the higher efficiency of a metal spring over a compressed air charge at giving back more of the energy invested in compression. This advantage arises from the fact that much of the work of compressing air is lost in heating the air, only to dissipate before the shot is taken, and thus wasted. Steel springs don't do that.
Consequently, by sharing the energy of the compression stroke between the air charge and a highly preloaded steel spring, more energy can be extracted from a given pump displacement volume, while simultaneously reducing the peak pressure of the charge. Hence more power from less effort: win-win.
The plot below simulates the result.