In that example, the mass of the operator accelerates in a reciprocating cycle to displace a crank, turning a screw in shear. That external force (torque, actually) is applied to the screwjack, which causes a spiral extension (or retraction). A screw exerts no force, it merely holds an extension, The cranked screw provides a mechanical advantage exceeding the proportional difference in mass. The mechanical advantage of the screw is complex but (disregarding friction and angle of attack) is very roughly a function of the proportional difference in the crank length and the radius and spacing of the threads. Very large effective lever arm -- but it all works by shear.