Cable-driven underactuated manipulators (CDUMs) have garnered increasing attention due to their characteristics of slender structures, high flexibility, and potential for miniaturization as well as modularity. To further improve the efficiency and reliability of CDUMs, additional efforts are needed to enhance the speed and amplitude of their stiffness variation. This paper presents two CDUMs, which are characterized by variable stiffness devices that utilize the principle of force amplification near the dead point position of the crank slider mechanism, along with a double-direction threading strategy. These CDUMs are expected to exhibit rapid and dramatic stiffness variation, as well as efficient and reliable actuation when working in conjunction with the proposed driving strategies. Kinematic analysis is conducted to further understand the mapping relationship between the cable lengths and the configurations of the proposed CDUMs.