  Ferromagnetic (FM) order in a two-dimensional kagome layer is predicted to
generate a topological Chern insulator without an applied magnetic field. The
Chern gap is largest when spin moments point perpendicular to the kagome layer,
enabling the capability to switch topological transport properties, such as the
quantum anomalous Hall effect, by controlling the spin orientation. In
TbMn$_{6}$Sn$_{6}$, the uniaxial magnetic anisotropy of the Tb$^{3+}$ ion is
effective at generating the Chern state within the FM Mn kagome layers while a
spin-reorientation (SR) transition to easy-plane order above $T_{SR}=310$ K
provides a mechanism for switching. Here, we use inelastic neutron scattering
to provide key insights into the fundamental nature of the SR transition. The
observation of two Tb excitations, which are split by the magnetic anisotropy
energy, indicates an effective two-state orbital character for the Tb ion, with
a uniaxial ground state and an isotropic excited state. The simultaneous
observation of both modes below $T_{SR}$ confirms that orbital fluctuations are
slow on magnetic and electronic time scales $<$ ps and act as a
spatially-random orbital alloy. A thermally-driven critical concentration of
isotropic Tb ions triggers the SR transition.
