The partitioning of the reaction force based on the extendedtransition- state natural orbital for chemical valence (ETS-NOCV) scheme has been proposed. This approach, together with the analysis of reaction electronic flux (REF), has been applied in a description of the changes in the electronic structure along the IRC pathway for the HCN/CNH isomerization reaction assisted by water. Two complementary ways of partitioning the system into molecular fragments have been considered ("reactant perspective" and " product perspective"). The results show that the ETS-NOCV picture is fully consistent with REF and bondorder changes. In addition, proposed ETS-NOCV decomposition of the reaction force allows for the quantitative assessment of the influence of the observed bond-breaking and bondformation processes, providing detailed information about the reaction-driving and reaction-retarding force components within the assumed partitioning scheme. (C) 2017 Wiley Periodicals, Inc.