Mode specificity in the cycloaddition of 1,3-dipoles to ethene and ethyne is investigated with the newly proposed Sudden Vector Projection (SVP) model, which attributes the promoting ability of a reactant mode to its projection onto the reaction coordinate at the transition state. The SVP model revealed that dipole bending and translational modes have large components in the reaction coordinate, consistent with the recent direct dynamics studies. It further identified several other promoting modes. The success of the SVP model is encouraging as it requires no dynamics calculations, and as a result it can be applied to many reactions. (C) 2015 Elsevier B.V. All rights reserved.