The ability to predict mode/bond selectivity and energy disposal is of central importance for controlling chemical reactions. We argue that the transition state plays a critical role in state-to-state reactivity and propose a simple sudden model based on coupling with the reaction coordinate at the transition state. The applicability of this so-called sudden vector projection (SVP) model is examined for several prototypical atom-triatom, namely, X + H2O (X = H, F, O(P-3), and Cl) reactions. It is shown that the SVP model is capable of qualitatively predicting experimental and full-dimensional quantum dynamical results, including those reported in this work, for these polyatomic reactions. These results, and those for other reactions, suggest that the SVP model offers a general paradigm for understanding quantum state resolved reactivity in bimolecular reactions.