  Recent experimental data on the $\Upsilon(4S)\to\Upsilon(1S)\eta$ and
$\Upsilon(4S)\to h_{b}(1P)\eta$ processes seem to contradict the naive
expectation that hadronic transitions with spin-flipping terms should be
suppressed with respect those without spin-flip. We analyze these transitions
using the QCD Multipole Expansion (QCDME) approach and within a constituent
quark model framework that has been applied successfully to the heavy-quark
sectors during the last years. The QCDME formalism requires the computation of
hybrid intermediate states which has been performed in a natural,
parameter-free extension of our constituent quark model based on the Quark
Confining String (QCS) scheme. We show that i) the M1-M1 contribution in the
decay rate of the $\Upsilon(4S)\to\Upsilon(1S)\eta$ is important and its
supression until now is not justified; ii) the role played by the $L=0$ hybrid
states, which enter in the calculation of the M1-M1 contribution, explains the
enhancement in the $\Upsilon(4S)\to\Upsilon(1S)\eta$ decay rate; and iii) the
anomalously large decay rate of the $\Upsilon(4S)\to h_{b}(1P)\eta$ process has
the same physical origin.
