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This paper describes a control strategy for throwing motion of the springed Pendubot based on the concept of unstable zero dynamics. An underactuated two-link planar robot called the Pendubot is investigated to realize dexterous actions of the superior limb in human throwing motion. A torsion spring is mounted on the passive joint of the Pendubot representing the flexibility of the cubital joint. In the proposed control strategy, the zero dynamics is intentionally destabilized when the end-effector of the springed Pendubot is constrained on a geometric path via output zeroing control for the deviation between the end-effector and the geometric path. The unstable zero dynamics drives the end-effector along the geometric path to achieve a fast and accurate throw in a desired direction when the input is devoted to constrain the end-effector on the geometric path. The unstable zero dynamics is analytically derived to guarantee the divergence of the end-effector along the geometric path. Numerical simulations confirm the effectiveness of the proposed control strategy.