A kinetic isotopic effect observed when a single substrate, in which the isotopic atoms occupy equivalent reactive
positions, reacts to produce a non-statistical distribution of isotopomeric products. In such a case the isotope effect will favour the pathway with lower force constants for displacement of the isotopic nuclei in the transition state.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1130