  The sum rules for the spin dependent structure function $g_1^{ab}$ in the
null-plane formalism corresponding to the moment at $n=0$ has been transformed
to the sum rule which relates the $g_1^{ab}$ with the cross section of the
isovector photon or the real photon. Based on these sum rules, we argue that
there is a deep connection among the elastic, the resonance, and the
non-resonant contributions, and that it explains why the sign of the
generalized Gerasimov-Drell-Hearn sum changes at very small $Q^2$.
