  We study the implications of the heavy-quark spin symmetry for the possible
spin partners of the exotic states $Z_b(10610)$ and $Z_b(10650)$ in the
spectrum of bottomonium. We formulate and solve numerically the coupled-channel
equations for the $Z_b$ states that allow for a dynamical generation of these
states as hadronic molecules. The force includes short-range contact terms and
the one-pion exchange potential, both treated fully nonperturbatively. The
strength of the potential at leading order is fixed completely by the pole
positions of the $Z_b$ states such that the mass and the most prominent
contributions to the width of the isovector heavy-quark spin partner states
$W_{bJ}$ with the quantum numbers $J^{++}$ ($J=0,1,2$) come out as predictions.
Since the accuracy of the present experimental data does not allow one to fix
the pole positions of the $Z_b$'s reliably enough, we also study the pole
trajectories of their spin partner states as functions of the $Z_b$ binding
energies. It is shown that, once the heavy-quark spin symmetry is broken by
means of the physical $B$ and $B^*$ masses, especially the pion tensor force
has a significant impact on the location of the partner states clearly
demonstrating the need of a coupled-channel treatment of pion dynamics to
understand the spin multiplet pattern of hadronic molecules.
