This paper describes a control strategy for throwing motion of the Pendubot based on the concept of unstable zero dynamics. An underactuated two-link robot called the Pendubot is investigated to realize dexterous actions of the superior limb in human throwing motion. In the control strategy, the zero dynamics is intentionally unstabilized when the end-effector of the 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. The unstable zero dynamics is analytically derived to guarantee the divergence of the end-effector along the geometric path. Numerical simulations and experimental results confirm the validity of the control strategy.