  Numerous exotic candidates containing a heavy quark and anti-quark (the
so-called $XYZ$ states) have been reported since the observation of the
$X(3872)$ in 2003. For these systems a study of the implications of the heavy
quark spin symmetry and its breaking is expected to provide useful guidance
towards a better understanding of their nature. For instance, since the
formation of the complete spin multiplets is described with the same parameter
sets, in some cases the currently available experimental data on the $XYZ$
states allows us to predict properties of spin partner states. To illustrate
this point we extract the parameters of the two $Z_b$ states by analyzing the
most recent experimental data within an effective-field theory approach which
treats both short-ranged contact interactions and the long-ranged
one-pion/one-eta Goldstone boson exchanges (OPE/OEE) dynamically. The line
shapes and pole positions of their spin partners are then predicted in a
parameter-free way and await to be tested by future experimental data.
