The study of hadronic transitions among bottomonium states, and their relative magnitude, can be used as a test bench for non-perturbative approaches to QCD. In particular, transitions through an $\eta$ meson, despite involving a heavy quark spin symmetry violation, have been measured to have an unexpectedly enhanced branching fraction with respect to those via a dipion system.
A set of recent results obtained using the data collected by the Belle experiment at the energy of the $\Upsilon(4S)$ and $\Upsilon(5S)$ resonances are presented. They include the measurement of the transition $\Upsilon(5S)\to\eta\Upsilon(1D)$, an updated measurement of the branching fractions of $\Upsilon(4S)\to\eta\Upsilon(1S)$ and $\Upsilon(4S)\to\pi^+\pi^-\Upsilon(1S,2S)$ decays, and searches for other possible transitions involving an $\eta$ meson.