There is a "spring" under the barrel that has a lobe sort of end which applies pressure (friction) to the wheel. The charge is loaded and the "gun" fired. The bore is covered and the force of the charge rotates the wheel which has a calibration marking of 1,2,3,4,5,6,7 and 8. A given quantity of powder could then be assessed as to it's "power". X grains that blows the wheel to 6 would be superior to the same amount from a different supplier that only moves the wheel to 3....etc etc etc and so on and so forth.
