I know that you can't always trust Wikipedia, so I just want to check something regarding their kinetic isotope effect page.
The first example implies that $\sideset{^{12}}\ C$ and $\sideset{^{13}}\ C$ can be used to determine whether a reaction is $S_N1$ or $S_N2$:
...In the nucleophilic substitution reaction of methyl bromide with cyanide, the kinetic isotope effect of the methyl carbon, in this case defined as $k_{12}/k_{13}$, was found to be 1.082 ± 0.008 ... The observed methyl carbon kinetic isotope effect is indicative of an $S_N2$ mechanism."
No explanation appears to be offered for this inference. I know that the reaction will not proceed via $S_N1$ because of the unstable carbocation but (how) does the KIE datum predict an $S_N2$ mechanism?