  A mixing of near-threshold quarkonium-like resonances with heavy
meson-antimeson pairs results in an enhancement of heavy quark spin symmetry
breaking, since the meson pairs are not eigenstates of the heavy quark spin.
The decomposition of $P$-wave states of meson pairs in terms of the
heavy-quark-pair spin states is considered in the channel with $J^{PC}=1^{--}$,
which is directly produced in $e^+e^-$ annihilation. Specific processes are
suggested for experimental study of the effects of the mixing with heavy meson
pairs and of the internal spin structure of bottomonium and charmonium
resonances.
