This is a criterion used in Rice–Ramsperger–Kassel–Marcus (RRKM) theory for locating the transition state. The reaction coordinate r at the transition state, according to this criterion, is given by ∂(Nɛr)/∂r = 0, where N ɛ r is the density of states at an internal energy ɛr.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 174 (https://doi.org/10.1351/pac199668010149)