  Understanding the spin structure of hadrons in the small $x$ regime is an
important direction to unravel the spin puzzle in hadronic physics. To include
spin degrees of freedom in the small $x$ regime requires going beyond the usual
eikonal approximation in high energy QCD. We developed an effective Hamiltonian
approach to study spin related observables in the small $x$ regime using the
shockwave formalism. The small-$x$ effective Hamiltonian incorporates both
quark and gluon propagators in the background fields and the background field
induced interaction vertices up to next-to-eikonal order. A novel feature of
sub-eikonal interactions is the background gluon field induced gluon radiation
inside the shockwave. Its relation to chromo-electrically polarized Wilson line
correlator is established both in small $x$ helicity evolution and in
longitudinal double-spin asymmetry for gluon production.
