  A light ($m_{\nu d} \lesssim $ MeV) dark fermion mixing with the Standard
Model neutrinos can naturally equilibrate with the neutrinos via oscillations
and scattering. In the presence of dark sector interactions, production of dark
fermions is generically suppressed above BBN, but then enhanced at later times.
Over much of the parameter space, we find that the dark sector equilibrates,
even for mixing angles $\theta_0$ as small as $10^{-13}$, and equilibration
occurs at $T_{\rm equil} \simeq m_{\nu d} \left(\theta_0^2 M_{Pl}/ m_{\nu d}
\right)^{1/5} $ which is naturally at most a few orders of magnitude above the
dark fermion mass. The implications of this are twofold: one, that light states
are often only constrained by the CMB and LSS without leaving an imprint on
BBN, and two, that sectors which equilibrate before recombination will
typically have a mass threshold before recombination, as well. This can result
in dark radiation abruptly transitioning from non-interacting to interacting,
or vice-versa, a ''step'' in the amount of dark radiation, and dark matter with
similar transitions in its interactions, all of which can leave important
signals in the CMB and LSS, and may be relevant for cosmological tensions in
observables such as $H_0$ or $S_8$. Minimal models leave an unambiguous imprint
on the CMB above the sensitivity of upcoming experiments.
