In an attempt to improve the performance of underactuated robotic hands in grasping, we investigate the influence of the underlying coupling mechanism on the robustness of underactuated hands to external disturbance. The coupling mechanisms used in underactuated mechanisms can be divided into two main classes based on the self-adaptive transmission used to route actuation to the degrees of freedom, namely single-acting and double-acting transmissions. The kinematic coupling constraint is always active in double-acting mechanisms, while there are specific combinations of external disturbances and mechanism parameters that render the constraint inactive in single-acting mechanisms. This paper identifies unique behaviors in terms of mechanism reconfiguration and variation in grasping contact forces that result from the underactuated hand’s response to external disturbance forces and show that these behaviors are a function of the coupling mechanism, actuation mode, and contact constraints. We then present an analysis of how these behaviors influence grasping ability of the hand and discuss implications for underactuated hand design and operation.