  Heavy-quark spin symmetry (HQSS) implies that in the direct decay of a heavy
quarkonium with spin $S$, only lower lying heavy quarkonia with the same spin
$S$ can be produced. However, this selection rule, expected to work very well
in the $b$-quark sector, can be overcome if multiquark intermediate states are
involved in the decay chain, allowing for transitions to the final-state heavy
quarkonia with a different spin $S^{\prime}$. In particular, the measured
decays $\Upsilon(10860)\to \pi Z_b^{(\prime)} \to \pi\pi\Upsilon(nS)$
$(n=1,2,3)$ and $\Upsilon(10860)\to \pi Z_b^{(\prime)} \to \pi\pi h_b(mP)$
($m=1,2$) appear to have nearly equal strengths which is conventionally
explained by a simultaneous presence of both $S_{b\bar{b}}=0$ and
$S_{b\bar{b}}=1$ components in the wave functions of the $Z_b$'s in equal
shares. Meanwhile, the destructive interference between the contributions of
the $Z_b$ and $Z_b'$ to the decay amplitude for a $\pi\pi h_b$ final state
kills the signal to zero in the strict HQSS limit. In this paper, we discuss
how the HQSS violation needs to be balanced by the narrowness of the
$Z_b^{(\prime)}$ states in the physical case, to allow for equal transition
strengths into final states with different total heavy quark spins, and how
spin symmetry is restored as a result of a subtle interplay of the scales
involved, when the mass of a heavy quark becomes infinite. Moreover, we
demonstrate how similar branching fractions of the decays into $\pi\pi h_b$ and
$\pi\pi\Upsilon$ can be obtained and how the mentioned HQSS breaking can be
reconciled with the dispersive approach to the $\pi\pi/ K\bar K$ interaction in
the final state and matched with the low-energy chiral dynamics in both final
states.
