  It is shown that most of the available data on the $pp\to pp\eta$ reaction,
including the invariant mass distributions in the $pp\to pp\eta$ reaction
recently measured at COSY, can be understood in terms of the partial-wave
amplitudes involving final $pp$ $S$ and $P$ states and the $\eta$ meson s-wave.
This finding, together with the fact that results within a meson--exchange
model are especially sensitive to the details of the excitation mechanism of
the $S_{11}(1535)$ resonance, demonstrates the possibility of investigating the
properties of this resonance in $NN$ collisions. The spin correlation function
$C_{xx}$ is shown to disentangle the $S$- and $P$-wave contributions. It is
also argued that spin correlations may be used to help constrain the
contributions of the amplitudes corresponding to the final $pp$ $^3P_0$
